Since My Disney Experience is going to be the end-all-be-all from now on, it's going to trump pretty much everything else out there. It's best to go ahead and bite the bullet now, and get rid of any alternate log-ins you might have floating around out there.
Determine what you want your ONLY Disney account to be from now on. If that's your DVC, you're pretty much done. Sign in to MDE and use your DVC info.
If you currently have two different log-ins... one for MDE and another for DVC... you're going to have nothing but frustration for years to come. So you need to consolidate to just one. I suggest defaulting to your MDE login, since that's what most dining and ticket info will be associated with. Call MS & have them reset your DVC account so you can use your MDE login instead of your old DVC-only login. They'll give you your club ID (this isn't your member number... something different) as well as a new one-time activation code.
Now... go back to the DVC site. Click "no DVC member account," then on the drop down, pick "I already have a Disney account." On the page that pops up, enter your current MDE log-in & password, as well as the Club ID and code that MS gave you.
Now, everything should be all linked up. If you log into DVC with your MDE info (which is now also your DVC info), you'll be able to see all your vacations. Log into MDE, and you'll be marked as a DVC member (just look under your MDE "my profile" near the bottom).
New DVC reservations might not pop up automatically, but if you enter in your reservation number and "claim" them, they should come over without any problem.
By the way... any trips that are already in progress (already made the reservation, linked it to another account, etc) may not be able to make the switch... but anything new going forward should work fine.
Another note, if you're a family who has multiple DVC log-ins or multiple MDE log-ins, that's fine, too. Just repeat this process for every person who has an account. Then, add that person as friend/family, select that they often travel with you, and send an invite to the email address that they use to sign into their MDE account. Once they log-in to MDE and accept the invitation (it's in "notifications" in the MDE drop-down menu that's just underneath the WDW search box), you'll see all of their stuff, and they'll see all of yours. When you make new plans going forward, you can note who will be attending while you're setting them up in MDE. Just make sure you're using the correct one if you have the same person listed multiple times. For instance, my sister was listed three different ways on my MDE account. If I link her over to a reservation, I have to pick the one that actually shows her Disney character, rather than any of them that has just the default Mickey icon. Eventually, once all of my upcoming trips and reservations are over, I can delete all the "ghost" versions of her from my MDE account and only the real one will be left.
Hope this helps out!