YOU HAVEN'T SCREWED UP YOUR SAVE IF YOU'VE DIED AND RELOADED!
I just got the achievement, and I died and re-loaded many times. You have to have downloaded the 'Ryan Industries Plasmids and Gene Tonics' download from the xbox live marketplace first. Then, before you defeat Fontaine, turn on 'Disable Vita Chambers'. So long as you haven't died and then saved it, this can be done whenever. I turned it on just before I entered the lift to Fontaine and I got the achievement. Huzzah!