They boost stats, but, yes, Pokemon get happier when you give them vitamins.
Also, the berries that reduce Effort Points also increase happieness (Pomeg, Kelpsy, Qualot, Hondew, Grepa, and Tamato). They drop EPs by 10 and not actually reduce stats permanently, so don't be shocked if a stat drops.
Also, the berries that reduce Effort Points also increase happieness (Pomeg, Kelpsy, Qualot, Hondew, Grepa, and Tamato). They drop EPs by 10 and not actually reduce stats permanently, so don't be shocked if a stat drops.