My 8 month old keeps sticking his fingers down his throat to the point that he vomits, how can i stop this?

First thought: The issue is disquieting but likely bothers you more than the kid. Some adapt such behaviors when they find it gets you to stop what you are doing and go to them. It is not adult like manipulation it is just their crude way of drawing attention. Try monitoring the issue out of your side vision, without looking at them directly. Even when checking for safety don't make eye contact. This may end it.