What stats are best to level up and which stats are not needed as much? I know Feena has low Vitality in second half of Disc 2 since she is not in the party after the Zil Shrine.

The best stat is for sure the WIT for more IP Speed and Action. BUT in fact all stats are important. STR for make more damage and HP/DEF if you don't want to be "OneShot By Baal ©" Probably the less important is Agl (but you need it too, just a few).

Anyway, Feena is more & more weaker during the Disc 2, Rapp have a powerfull SP Attack without ACT Charge, Liete have a lot of manapoint + A 100% Cancel SP Attack. Feena is just here for heal partners.

What Etsuna said, Feena basically just drops off more and more as the game progresses. When she joins your party she's by far the most powerful member due to starting with magic and getting Burnflame early, and then as you progress through the game the other characters just gradually outstrip her due to having better Skills and stats. She'll eventually end up being an almost entirely support character.