Joseph…
The Electoral College system varies by state. If you vote Trump in CT you are vote for a slate of electors pledged to vote for Trump and his running mate. You are not voting for electors who are free to cast their ballots in December for another person.
In some states the delegates to the Electoral College are free to change their votes, but it is very rare that it happens.
As for a replacement VP candidate, it is not the same matter. No one votes for VP or the electors for VP, the Vote is for a particular Presidential candidate and his/her running mate. One can’t vote R for Pres and D for VP.