User blog comment:Icy44266/Why do my Loyal Squad members die.../@comment-737801-20110109061219

Probably because you didn't either pick an ideal/loyal second squad leader. If the second squad leader isn't Garrus, Jacob, or Miranda, the specialist sent into the Vents will die no matter if they are loyal or not.

See: Mass Effect 2 Guide and Collector Base: Infiltration.