In my mind the problem with trying to do DCA between two different investments raises multiple problems.
Ideally, you want to sell high and buy low. If they're invested in similar products, that is going to be extremely difficult to time, and by trying to time it, you'll likely lose money.
I would just roll the entire amount over to the new account with Vanguard whenever you make the decision to do so. If anything, you'll quit paying the higher fees sooner, which will save you more money than timing will.