Pass C++ vector of unique_ptr

I have a class Outer which contains an Inner member and owns a vector of unique_ptr elements:

using Elements = std::vector<std::unique_ptr<Element>>;

class Outer
{
    void call()
    {
        _inner.aMethod(_vec);
    }

    Inner _inner;
    Elements _vec;   // This should remain the owner of each Element
};

Inner receives the vector of unique_ptr elements and it transfers ownership to it's own vector class member:

class Inner
{
    public:
    Inner() = default;
    ~Inner() = default;

    void aMethod(Elements& vec)
    {
        _vec = std::move(vec);
    }

    private:

    Elements _vec;  // This is a vector of unique_ptr but I don't want this class to own the memory
};

I stupidly used std::move() because otherwise the compiler complained I was trying to call a deleted function (probably the copy constructor) on each vector element.

I have an illegal memory access and I believe it is because both classes think they own the vector elements and one has tried to delete an already-deleted Element.

How do I have Outer owning the memory and just pass the elements to Inner to use (not take ownership)?