From the bodybuilding judges pointof view its when they pop out.
BUT, vascularity is actually the growth of the veinous support system. (Mabye arterial too, I cant remember). And what causes that is strenuous work of the muscle. More into the cardio realm... I think that old time bodybuilders experianced increased vascularity by doing high rep work precontest, and that caused the commonly accepted practice of light weights pre-contest.
Physically fatiguing a muscle makes the body more apt to create a better support system to bring blood more efficiantly into the area and more efficiantly remove waste by products.
But hey, whaddo I know?