The "Springfield migrants ate my dog" thing is wild. It went from a thing Vance had tweeted yesterday that I was joking about with an Ohioan friend to the headline from Trump's debate performance.
That said, I don't think it's necessarily decisive for his supporters if Trump lies or appears to be incoherent. If he comes across as weak however it'll kill his chances. Those who vote for him gravitate to the appeal of a partisan power acting on their behalf.
Vance turns out to be damaging in this regard, he lacks charisma and conviction as well as Trump's impenetrable arrogance—I got blindsided by how poor the guy is as a "retail politician". Pence in "Christian Dalek" mode declaring he wouldn't dine with a woman other than his wife was better for Trump.