This VP debate was not the usual sideshow VP debates are: it was also meant to evaluate two candidates, one of whom could well end up becoming a president.
On positions, of course each one would appeal to their party. But a potential president needs to command respect. Pence never was very presidential: he seemed to be just too good as a lackey. But if TP were to die, he could have somehow reconstructed himself. A VP in a suit speaking platitudes was all he needed to be at the debate, a blank to be filled in with the image of a president.
But now? The fly that landed on him for two minutes made it clear how easy it is to laugh at him. Once cast in a joke, he was an obvious natural fit. That little dot on his head was enough to topple his image.
Being awful still gets respect. Being ridiculous does not. If he campaigns in the next three weeks, people will remember the fly. If TP dies and Pence campaigns for himself, he will be the man under the fly. This was the most memorable thing about him.