If Nixon had picked Volpe as his running mate, Gerald Ford would never have been President—Volpe would have succeeded when Nixon resigned. Whether he would have been able to win the 1976 election, who knows?
I think Volpe would’ve had serious problems as an Establishment liberal (although Agnew himself had been one, but he moved rightward as Governor, especially due to the rioting and social breakdown in 1968 - but Volpe did not move to the right). Volpe almost certainly would’ve lost the 1976 primary to Reagan unless he was as politically nimble as Ford was in getting Rockefeller off the ticket in favor of Bob Dole (although Reagan erred greatly in picking liberal Republican Senator Dick Schweiker as his running mate).