Accessing a private vector

I'm new to C++ and struggling to setup a proper class with private members and accessing them. Basically, I have a vector of Layers which make up a Stack. I'd like to create a simple function which simply adds a layer to the stack. I've tried to simplify this example to explain my problem.

// Stack.h
namespace NS {
    class Stack
    {
    public:
        Stack() {

        }
        virtual ~Stack() {

        }

        std::vector<Layer> const &getLayers() const;

        virtual Layer* AddLayer(TextureBase texture);

    protected:
        std::vector<Layer> _layers;
}

This is my cpp file

//Stack.cpp
namespace NS {

    std::vector<Layer> const &Stack::getLayers() const { 
        return _layers; 
    }

    Layer* Stack::AddLayer(TextureBase texture) {
        Layer* newLayer = new Layer();
        newLayer->setTexture(texture);

        std::vector<Layer> layerStack = Stack::getLayers();
        layerStack.push_back(*newLayer);

        return newLayer;
    }
}

In my main file I create the stack and then try to add the layer like this:

auto myStack = getStack();
myStack->AddLayer(myTexture);

However, when I place a breakpoint after this line, myStack doesn't contain any layers (the size is 0). I can step through the AddLayer function and it does appear to add the Layer to the Stack... but perhaps it's not referencing the vector correctly. Can anyone provide some guidance as to why this is occurring?