He won't improve as a weak side platoon player. Even if your arbitrary 50/50 odds are accurate, that is way higher than what they would become with Vaughn sitting out most games waiting for a favorable lefty pitcher match up.
As for the middle paragraph, this isn't what stats are for. They are a snapshot in time in a dynamic game. A guy hitting .333 isn't getting a hit every 3 ABs, and shouldn't be expected to do so going forward. This even more important when looking at talented young players who are going to whip between extremes just by their very nature. Literally no one is going to put up the exact same ratios of their stats for the rest of the season, so mapping out the season based on that is an exercise in futility.