Ralph Called achievement in Metal Gear Solid HD Edition: 2 & 3

During Operation Snake Eater, press start, then press RB to access the viewer, or go into the CURE menu. From there, use the right stick to spin Snake several times, before unpausing. Snake should vomit.

Eating poison or rotten food also does the trick.

Keep in mind that vomiting will deplete about 1/4 of your stamina bar, depending on your difficulty level.