Critical thinking

This is a guess, and I am assuming the victim knows which vial contains which liquid:

1 vial of water in one box; everything else in the other.

Why? Because for box #1 there's a 100% chance that all will be swell for the victim. And in box #2, there's a 5/12 chance that everything will be okay (e. g. 41.67%). Average the two and you get 70.835% odds of a good outcome. Not too shabby.

If you even out the vials, you have a 50% chance from both boxes, the average of course is 50% and so it's not as good a shot.

If you place two waters in one box and the remainder (with the poison) in box #2, you still have 100% in box #1 but now box #2's odds are 40%, and the average is obviously worse.

If you place two waters in box #1 with 1 poison, and the remainder in #2, you get a 66.67% chance in box #1 and a 33.33% chance in #2, with an average of 50%.

3 waters and 1 poison in #1 gives you a 75% chance, with a 37.5% chance in #2, averaging 56.25%.

4 waters and 1 poison in #1 yields an 80% chance there and an 18% chance in #2, for an average of 49%.

That would be my answer as well