Rockefeller would not be able to win the South, not even with Reagan on the ticket - simply because he was Rockefeller. In 1968, Wallace may do even better than in OTL - getting OTL Nixon votes, though Humphrey may lose enough liberal vote to Rockefeller to balance it off. I don't really think Reagan would accept it though. He has just been governor for 2 years. And if he would like to be a future contender for the presidency, it's better for him to stay in California than to go to Washington DC. Due to his fundamental differences with Reagan, Reagan would not be able to achieve anything - unless he would like to become a thorn in the Rockefeller administration. No matter what, after wasting 4 or 8 years in the White House without anything to do, he may not really be a serious contender in 1976, and he surely won't get Rockefeller's blessing. However, it's worth noticing that given the stress of being President, Rockefeller may well have died of a heart attack say in his second term, leaving the presidency to Reagan

And then, you have an earlier Reagan presidency that at least lasts until 1981. Anyway, like what I said at first, I really don't see Reagan accepting the VP slot. No.