> I make a newtype, implement Serializable on it, find-and-replace all Vec3s with my own SerializableVec3, fix about 500 errors one by one by adding .0 everywhere, implement wrappers for all the Vec3 methods I want to use, rinse and repeat for every other foreign type I want to serialize.

You could also implement Deref (and, if appropriate, DerefMut) on the newtype, though there seems to be controversy over this pattern; for the specific case of a newtype that exists for the specific purpose of enabling foreign trait implementation, it seems to be a fairly straightforward and effective way of dealing with things.