You could, but I wouldn't recommend it. Aqua Ring only helps vs residual damage, and its targetted damage that's going to threaten bulky waters most of the time, unless they're forced to switch in and out constantly without recovering.Can I drop Protect on Vaporeon and use Aqua Ring? Also Aqua Ring for Milotic. It;s vesy easy to predict Protects and set-up, so it isn't really worth it...
Protect on Vaporeon is because of Wish, if you don't have protect then it's pretty easy for a lot of threats to beat you, when Wish+Protect would have stopped them. If you want to run without Protect on Vaporeon, either run Surf/Ice Beam/HP Electric for better coverage, or two of those attacks + Toxic for stalling and building up damage more quickly
On Milotic I'd recommend neither Wish or Protect, but a third attack or a status move like on Protectless Vaporeon.