Yes, that's what I am implying in this theory/possibility based on the facts that Nomura choose one of his answers in the Ultimania to point out that Vanitas is different from Xemnas and Ansem SoD and has not (yet possibly) a physical form unlike them.
That could lead to him not being one of the 13 right now but in the near future, although that still doesn't bring us any closer to guess the six remaining darknesses.
I think so far as "confirmed" can be counted:
1. Master Xehanort
2. Young Xehanort
3. Xemnas
4. Ansem SoD
5. Xigbar/Braig
6. Saix/Isa
leaving us with seven seats to fill, regardless if Vanitas will be eventually the 13th or already fills in one of the seats 7 to 12, I think the chances are high that he will be one of the 13 in the end as he exhibits the golden "Xehanort-seed"-eyes and was practically
alone with MX for four freakin' years.
The main issue remain the six other seats to be filled.
According to what Xemnas explained in-game I would assume we can rule out Demyx and/or his original persona (too weak and unsuited), and surely also Marluxia and Larxene (they fit into the Traitor-category just as Axel and Roxas do according to Xemnas).
Luxord now is an interesting case. The few tidbits we learned about him painted him in a rather laid-back attitude and loyal, but also sophisticated and with a certain power-level. Judging by all we came to know about him, I would count him as a candidate for one of the 13, albeit more on the same boat as Isa as Braig, Young MX, Ansem SoD and Xemnas seem to be the only ones who really "willingly" go along with MX.
That would make 8 then:
7. Luxord/???(his original persona)?
8. Vanitas (high possibility)
That still leaves us with four unknown darknesses.
One I could surely imagine (at least until the real final battle starts) is Terra, or rather Terra-Xehanort.
9. Terra-Xehanort.
The truly difficult ones would be the last three, as Dilan, Even, Ienzo and Aeleus got correctly reborn together with Lea so I doubt they are part of the 13.
Hello and welcome to the depths of the mess that is the KH-section, lol.
Be sure to stay afloat.

Any "list" of the seven is speculation right now anyways, so no one can claim to have the "correct" list yet. It's typical for Nomura to give us something like this and he's probably out there and (laughs) at all the wild mass guessing going on. You're the first one I see who included Xion in the roster though, lol.
---
Speaking of it, I have not spilled out my take on the whole stuff yet despite having thought about it often (probably because I was so busy the last month).
So here we go (Warning for monster post with color-coded names, lol):
Possible 7-Light constellations:
Candidates (color code):
1.
Terra (brown)
2.
Lea (red)
3.
Ventus (green)
4.
Xion (gray)
5.
Roxas (gold)
6.
Riku (dark green)
7.
Sora (blue)
8.
Aqua (light blue)
9.
Kairi (pink)
10.
Mickey (light orange)
10 candidates for seven spots.
Set Characters:
Sora,
Riku,
Mickey,
Aqua and
Ventus.
Variable Characters:
Terra => due to no ensurance that he can be snatched away from the 13 before the final showdown. Maybe he is the last one to be saved and the 13 darknesses have to be disrupted beforehand. I personally doubt that he will be still on the enemies' side in the very final showdown, but in the climax battle beforehand there might be a substitute needed.
Lea => A Keyblade doesn't mean a automatically a light of the X-blade, and Nomura indicates in the Ultimania he gave Lea mainly a Keyblade in order to give him the power to bring back the people he wants, namely Isa, Roxas and Xion, the people most important to him.
That of course doesn't
rule him out as a light so he has to be included.
Kairi => was summoned by Yen Sid because she can wield a Keyblade. Since she's
also one of the seven pure lights which function as the
very foundation of the current universe she's for sure targeted by Xehanort to be destroyed or to be used to re-open the Door to Darkness. So putting her on the front lines directly is probably a bad idea => doesn't neccessarily makes her directly
also a light of the X-blade. Still, her
receiving Keyblade-training can come as a nasty surprise for Xehanort when she's able to thwart attempts on getting her and protect the other princesses in turn as well. Kairi nonetheless has to be counted as a candidate anyways as Nomura specifically included her in the list and the secret ending made a point by Mickey talking about the missing protector just as Riku brought her in.
Roxas => was shown independently from Sora and Ventus in the opening (just like in the group shot in-game when Sora causes Xigbar to freak out), hinting that his greatest wish of becoming a person on his own might get fulfilled.
The explanations of Ansem the Wise, the probability of him having developed a heart, Lea's promise to "bring them back" (this can double for both Roxas & Xion) and Sora himself
wanting for Roxas to exist by himself give further hints into that direction. Furthermore, Roxas, being originally created from two lights (Sora & Ventus) might have become one himself by development.
Nonetheless, all this is still theory and it has furthermore to be pointed out that Roxas can become an independent entity again and even help out in the final battle, just like Donald and Goofy,
without having to be one of the seven lights of the X-blade!
Xion => The oddball, never shown either in the secret ending nor in the opening, but might still have similar pros and cons like Roxas, with the exception
of her not being "born" directly from Sora and Ventus like he is, which may put her chance a level below Roxas, but she nonetheless can also be,
just like Roxas, Donald and Goofy, an allied fighter without having to be a light of the X-blade.
Likely combinations:
I:
1.
Sora
2.
Riku
3.
Mickey
4.
Terra
5.
Ventus
6.
Aqua
7.
Roxas
(DDD Opening roster)
II:
1.
Sora
2.
Riku
3.
Kairi
4.
Mickey
5.
Terra
6.
Ventus
7.
Aqua
(DDD Secret ending roster)
III:
1.
Sora
2.
Riku
3.
Kairi
4.
Mickey
5.
Lea
6.
Ventus
7.
Aqua
(DDD Secret ending roster Alternative A: Lea substitutes for Terra as he's with the 13)
IV:
1.
Sora
2.
Riku
3.
Kairi
4.
Mickey
5.
Ventus
6.
Aqua
7.
Roxas
(DDD Secret ending roster Altenative B: Kairi substitutes for Terra as he's with the 13)
V:
1.
Sora
2.
Riku
3.
Mickey
4.
Terra
5.
Ventus
6.
Aqua
7.
Lea
(DDD Opening roster Alternative A: Lea replaces Roxas)
VI:
1.
Sora
2.
Riku
3.
Mickey
4.
Lea
5.
Ventus
6.
Aqua
7.
Roxas
(DDD Opening roster Alternative B: Lea replaces/substitutes for Terra)
This is already a monster post so I didn't include the "Xion"-options but it isn't that complicated, as Xion could also serve as a substitute for Terra or
replace Roxas from the Opening roster just like Kairi and Lea can.
As another information, I personally think it is perfectly possible that the seven lights do not stay the same all throughout KH III but that there may several
of these combinations
temporarily active.
Speaking of this, another scenario I can imagine is that the BBS-Trio might be saved last out of all due to forced circumstances like the 13 Darknesses attacking the worlds and the Princesses, leaving Sora, Riku and Mickey no time to leave the RoL to search for Aqua and Ventus therefore can't be reawakened (we also don't know if his heart is already fully healed). Terra is apparently on Xehanort's team so if the BBS-crew is only rescued/restored in the second half/last third of KH III, we might need
three temporary substitutes for the first half of the game who later serve as normal allies when TAV take their rightful places as X-blade lights.
That would be this:
VII.
1.
Sora
2.
Riku
3.
Kairi
4.
Mickey
5.
Xion (serves as temporary substitute for Aqua)
6.
Lea (serves as temporary substitute for Terra)
7.
Roxas (serves as temporary substitute for Ventus)
As for my personal preference on what the final roster one shall be, well, while I can accept them all as long as the non-light-characters get their fair share of
ass-kicking as well I would prefer either combination I. (regular Opening one), II. (Regular secret ending) or IV. if saving Terra can only be done after the
climax clash.
You don't have to be a light of the X-blade (one of the seven lights) in order to kick ass or otherwise use your Keyblade.
We have more Keybladers than spots to fill anyways (see above).
Being the troll he is I could also imagine Nomura not showing the Unversed at all in order to keep the involvement of Vanitas uncertain just for the lulz.