Have you noticed that several top Democrats have taken their names out of contention to be Kamala Harris' vice presidential nominee? A quick web search finds the following three at one site dated nearly a week ago: Gov. Wes Moore (D-MD), Sen. Joe Manchin (I-WV), and Gov. Gretchen Whitmer (D-MI). A more recent dropout is Gov. Roy Cooper (D-NC).
All have given noble, selfless reasons for "declining the honor." None should be automatically believed, successful politicians are ferociously self-centered people. If they don't want the "honor" of being on the 2024 Democrat ticket, it signals they are far from certain Harris can be elected and they don't need the L on their resume'.
If Harris should win, her VP gets to spend 4-8 years going to foreign dignitary's funerals and presiding over the Senate - not a terribly exciting prospect. The odds a youngish, healthy woman is going to drop dead in that period of time don't make the VP job a good bet. Vance has much better odds, and would only have to serve 4 years in the job in any case as Trump term limits out in 2028.