What's the question? Whether you can draw vertex arrays?
VertexArray doesn't specify whether it can (or even should) be type cast to that.
Yes, it's written in the documentation. You can access a pointer to a contiguous array using the index operator.
But it's easier to just use the
void RenderTarget::draw(const sf::Drawable&, sf::RenderStates)
overload. There's a code example for exactly that, and the tutorial is even more verbose about it...