It's not an understudy position. The role of VP has historically been a way to "balance the ticket" between factions in the party. So, a Kennedy from Massachusetts and Johnson from Texas. Or California's Reagan with a Connecticut Bush.

More recently, the VP has been a means of whipping votes in the House (Cheney and Ford) or the Senate (Gore, Biden, Pence) and raising money from affiliate donor networks (all of the above, but Harris and Vance more than ever).

If you want a Presidential job training program, look to the governor's mansion or the State Department. But by the time you're VP, you're not training. You're in the game.