Alright, Tate decided to do a post about how you decide which Viv you should keep, it's my turn to help with the ivs part :3. Here are the best stats you can have for a level 12 vivillon with a timid or modest nature. Numbers in the parentheses are the ivs for that stat:
Timid
HP: 44 (24-31)
ATT: doesn't really matter tbh (xx)
Def: 20 (25-31)
SPATT: 30 (29-31)
Spdef: 20 (25-31)
Speed: 33 (31)
Modest
HP: 44 (24-31)
ATT: doesn't matter tbh (xx)
Def: 20 (25-31)
SPATT: 33 (29-31)
Spdef: 20 (25-31)
Speed: 30 (31)
These stats do not mean you have perfect ivs, just the best range of ivs. I will edit in the IV ranges in a sec.
Hope this helps :).
Edit: Lockie, you stole my idea ;-;.