Add vector_type to is_vec, because vector_type is private
and it couldn't be used in cast. Besides if not is_vec, you can't
expect to find vector_type. The conditional in the cast return type
works like if V2 is not a Vec, you can just use V as the vector_type,
anyways the first is_vec is false so the whole condition is going to
be false anyways, even if it results that V == V.
