How do I move a unique pointer from one vector to another vector of unique pointers?

Well, in that case you have two conceptually independent operations:

1. Inserting an element into a container. As you want to obliterate the source (which is actually necessary because std::unique_ptr is a move-only type), use std::move to enable move-semantics.

   destination.emplace(destination.begin() + m, std::move(source[n])); // or .insert()
   

2. Removing the plundered element from the container.

   source.erase(source.begin() + n);