But honestly, if it were any other character, I'm sure the link would've been made. I'm sure that if Mickey ever met Roxas before seeing him in the Re: Coded ending, we'd be a lot closer to finding Ven.
And just as you said, Xigbar/Braig met Ventus for about the same amount of time that Donald and Goofy met him for. So yeah, I guess at the end of the day it really just comes down to a matter of intellect. AKA Donald and Goofy are dumb af case closed <3
It really depends on having more than just an one-time contact or "seeing from afar" thing in play. As the example brought up with Kairi, for the "link" to be made it would have to have be a character who actually has a sort of deeper bond/connection with Ventus, and those are arguably only characters who either a) aren't around anymore by the time Roxas is a thing (Aqua, Terra, Eraqus, possibly Zack or Hercules) or b) do not have any (or not enough) connection and meetings with Roxas. (Mickey, arguably also Hercules as he never truly sees him either).
Xigbar/Braig is a different pair of shoes mainly because of a) he's working with Xehanort, b) got privy to Ventus' apparently very memorable death glare (which D & G didn't get) and c) is meeting and interacting with Roxas on an almost daily base.
Even Axel/Lea
did not remember anything
despite being with Roxas almost every day too, and why is that?
Because just like Donald and Goofy Lea only met Ventus for a very short time and
forgot about it ten years later.
I'm somewhat getting the impression that you're giving too much in-universe importance to these short scenes in BBS which were nothing but fanservice from one end to the other primarily and secondarily serve for Ventus to either learn an aesop or his next destination. The "physical" link of the BBS trio (as opposed to the metaphysical one of Ventus/Sora and Terra/Riku, Kairi and Aqua do not really have a true "connection" in the usual sense of leaving Kairi out or as an afterthought) is Mickey, he's the one who befriended at least both Aqua and Ventus and who is, as shown by the scenes in the white throneroom in DDD, still bitter about not being able to save them from their grisly fates.
And this link IS followed further in the series by having Mickey learn in Re: Coded this crucial piece of information that may give him a hint as to where Ventus' heart is.
Sora has never physically met Ventus and would probably call him Roxas upon 'meeting' him the first time UNLESS his heart 'tells him' that his name is Ventus.
If Riku can tell them apart in less than five seconds
without having a significant connection to either Roxas or Ventus I'd say Sora would not have much of a problem in this regard since he's closely connected to
both of them.
I'm just wondering now if Sora's been informed on the situation since Re:coded, as the discussion going on made me remember Mickey and Yen Sid do have their suspicions there now.
As far as we know Yen Sid and Mickey did not tell Sora anything more specific than what we actually witness in the mementos in Dream Drop Distance.
Had they outright told him that the heart of another person, in this case Ventus, is sleeping
directly inside him, the manipulatons and confusing statements of Xemnas, Xigbar and Young Xehanort would not have had such an impact on Sora and he probably wouldn't have been duped so thorughly.
I.e. when Xemnas comes with the "box's label matching the content stuff" in the Grid Sora could have just countered with "I know I have another, injured heart inside me that needs to be saved, and I'll work towards this myself, so your petty mindgames won't work."
The whole "your heart's a prison"-shtick from Young Xehanort probably would not have worked at all had Sora known about Ventus' more "direct" connection to him beforehand.
Or it would have at least made for a potentially more interesting back-and-forth banter between YX and Sora instead of the latter just being the confused puppy.
But of course this wouldn't have had the same "dramatic" and "mysterious" effect in terms of plot demand.
As to why Mickey and Yen Sid didn't take Sora aside for a more "personal" briefing on such a potentially huge special issue one can only speculate.
They were certainly not expecting Xehanort to interfere with the exam, that much the game made clear, so maybe Mickey planned to brief Sora on this after the exam is over and done with which was drafted in the first place to let Sora and Riku learn the powers needed to awaken sleeping/lost hearts.
This would make sense with the recent development in 0.2 when Yen Sid explains to Riku why they haven't spoken about Aqua beforehand, because then if not Riku
Sora certainly would embark on an unprepared and reckless, instinct driven rescue mission.
Based on this, knowing Sora one might guess how things would(or will) turn out once Sora finally learns what the
whole deal about Ventus is and since it has somewhat of a personal drift to it his reaction might turn out even more extreme (the way he reacts when witnessing Roxas' torment firsthand by seeing his memories in DDD may already be a sort of appetizer to it).
On the other hand it may of course also be that Yen Sid and Mickey simply neglected to tell Sora due to plot reasons (aka Xehanort's plan must again play out nearly flawlessly thus the plot needs important information withheld from Sora. 0.2 punches almost in the very same wound again by having Sora arrive only
after everything is said and done. Riku and Kairi get some important info, but Sora again misses out on it).
Instead, you can edit your post to include things you might have forgotten to talk about in your original post! 
