I'm not personally familiar with Vedanta or emanationism myself so I can't properly answer your question. Based on your description though I don't think they're necessarily the same idea. Spinoza imagines a single fundamental matter that all things are constructed from in their own time and place. He does not posit a single fundamental truth that our realities more or less pertain to. A chair with a wobbly leg isn't less of a chair in non-essentialist thought whereas essentialist thought would say it is further away from the singular 'truth' of what a chair is because it embodies the essence of what a chair is to a lesser extent. (This shit is why essentialist thought can never be liberationary imo, it implicitly posits a hierarchy as a transcendental truth)