I keep screwing up my vayne adc and I don't know why or how? Help.

I've been playing vayne adc for some time now and I keep screwing it up. I can cs quite decently with any other champ but vayne is so hard to last hit with (at least for me) so I end up with fewer cs than I should. Another thing is lane control, I feel like I either push too hard or let them push back too much and almost always end up losing my turret first. Sometimes when I do get a lead on my lane I end up getting ganked constantly or a 4 bot fiesta and just get crushed with the camping. I don't know what to do exactly. I want to take down the enemy turret fast but not too desperately enough that I end up dying too much to ganks and cs well. I just don't know why I always lose the first turret. I feel like I play too passively cause I get poked down easily. Anyone have any suggestions or tips cause I have no idea what I'm doing wrong