We know the Heartless were born because of Experiments which all but forced the Darkness into the Heart. If Ven's Heart was pulled out of his Body, again, much like Riku's Heart was pushed out of his Body, even if Heartless were still around - which they weren't until a year after BBS, by when I'm assuming the Ven-Sora relation was already in place - Ven wouldn't be in any danger of becoming one.
And Nobodies aren't even always born when the Heartless are. I'd sooner wager that if Ven's Body wasn't Erased or something, that it's still out there somewhere, lifeless, like KH1 Kairi.