The face made in the final seconds before you ejaculate. It looks like someone is holding a cup of vinegar under your nose. The few seconds when you are unable to stop humping your partner even if it means life or death because you are about to bust a nut.
1.Tom was sleeping with a married women. He was sticking it to Mary when her husband kicked in the bedroom door. Tom couldn't run away from Mary's husband because he was making his vinegar face.
2.When Joe was making his vinegar face he looked like a wild donkey getting circumcised.
The facial expression during sex that coincides with the last thrust before coming.
Imagine the contorted 'fuck face' being pulled by the male as hes about to come, and compare to the wincing one would experience when sucking a lemon, or tasting vinegar. Hence, the vinegar stroke.
Ah man, I was banging this chick, and I was up to the vinegar stroke when my phone rang. Ruined the moment.