I copied this from Bulbapedia, about the mechanics of Transform...
Transform changes the user's current type(s), current stats, current stat levels, current move(s), current species, current cry, and current catch rate to that of the target's.
Transform does not copy the target's status ailments, level, current or maximum HP, base experience, palette colors, or the generic minimize image.
I'm not really sure yet if using Ditto works, because I haven't tried yet since ever.
Although you could try it, to see if it works. Remember, HP can't be copied.
Maybe the only possible way to see if Victini's HP IV is perfect is to use a Level 58 Poke with Night Shade/Seismic Toss against Victini.
IIRC, when HP IV is flawless, then HP is 59.
If it survives, then it more likely has 31 HP IV...although the IV could be from 27-30, if not 31.