It's the presence of nutrients that stimulates enzyme production. When the pitchers first open, they are charged with enzymes. If no insects are caught, enzyme production decreases so the plant can reduce energy output. If insects are caught, enzyme production increases.
As the insect struggles inside the pitcher, it releases metabolic compounds that the pitchers detect. Dead insects are covered with these compounds because they struggled before their death (however that might have occured).
I've found that VFTs will accept dead insects if the trigger hairs are stimulated. If the trap presses tightly together (this takes a while), then the plant will most likely digest the insect. Mechanical stimula is what causes the trap to close. However, it's the presence of nutrients that stimulates the enzyme production, just like in nepenthes, and all CPs for that matter. Place a dried bloodworm on a Sundew leaf, and the leaf will curl up and produce digestive juices even though the bloodworm didn't move while on the leaf.
Brian