Most voters would say either "if Joe isn't up for the job as President, why is he now in a job which is mainly to be the new President should the President leave office for some reason"... and the other one is "Why should we be like Russia which kept switching the role of Putin to some other position and put a Putin puppet into the Presidency while we all know Putin is the real dictator".
And they would be correct.
Second there isn't any real benefit of having Joe on the ticket at all... because his age and possible mental decline would be in sharp contrast to JD Vance. We now have an advantage with a ticket of a 59 year old Presidential pick and a probable near same age vice Presidential pick with WAAAY more experience than JD Vance... who has served as a Senator for 2 years and before that... nothing expect jobs that Peter Thiel got for him in silicon valley.
So... who does the public want? Trump ( captain chaos and a questionable mental stability not to mention intelligence factor, proven record of committing multiple felonies, probable traitor to the US in foreign affairs, etc, etc, etc ) and JD Vance (owned and operated by a specific extreme libertarian billionaire) or a ticket of Kamala Harris, former Prosecutor, former Senator, now former Vice President in a term that saw enormous investments in America's future... and a likely experienced solid Democratic Senator or Governor with a track record in both politics and/or military service.
The choice should be an easy one for even the most "center right" voter with half a brain.