Joshua Primo happens to be the youngest player in the NBA but hasn't played like it thus far. That's not to say he's entirely polished, but he's shown plenty of poise and confidence for a player who's barely 19. Given that he’s still a teenager, his ability to hold his own against other NBA players suggests that he has a fairly high ceiling.
Still, San Antonio has brought him along slowly, but with White and Bryn Forbes gone, he’ll now see consistent minutes over the remainder of the season. He'll also have the ball in his hands more with White gone, helping Dejounte Murray and Tre Jones with playmaking responsibilities.
That will definitely be a learning experience considering he played mostly off-ball in college, but he’s already shown a knack for it.
While he's listed as a shooting guard, the Spurs' plan for Primo could be to eventually have him toggle between both guard spots. In this case, he could develop into the primary playmaker on the second unit and also play alongside Murray as a secondary creator.
Couple that with the possibility of Primo becoming a high-volume 3-point sharpshooter and that he's 6'6 and probably still growing, and he should turn into a fairly unique player. Also, his size and length should help him evolve into a good defender who can guard multiple positions.
So who has the higher upside? While both are definitely talented prospects, I would give the edge to Vassell. He has significant offensive potential and has skills that have yet to be fully utilized. He's also a terrific defensive player and still has a ways to go before peaking on that end of the floor.
As for Primo, I feel that his ceiling is White 2.0. That's a great player, no doubt, but he probably maxes out as a very good sixth man or an above-average starter. Vassell's ceiling could be higher than that. Therefore, between Vassell and Primo, Vassell has a higher upside.