personally, i would give a poisonous talent at levels 3, 5, 7, and 9, because Brew Poison seems like a less immediately useful thing to gain than sneak attack, and this way you're getting something really useful at each level, if you know what i mean.

also would make improved formula changed to have the DC's increased by only your Intelligence modifier - he's coming from the assassin class right? intelligence helps with the death attack DC and increases your craft skill as well, so i'm not sure why you want it wisdom. i took out the "increased by half of his Secret Vial level" because i think that's too much.