They will roughly split the union vote, Trump's problem are Haley voters the older ancestral, moderate, pragmatic republican voter that turned out for Haley in the closed GOP primaries in Mi and WI by 15-18%. They despise MAGA and everything Trump.
Trumps other massive problem is women. He is losing women by 24 points. Having Swift and Beyoncé, it's all about women and reproductive rights. That issue is crushing Trump, picking Vance did nothing to help.
I hear Trump is so desperate he is negotiating with Haley for a few stops in battleground states, he is bleeding GOP support from 2020, he can't afford to lose any 2020 votes.