Obviously, the mouse is PHYSICALLY shaped different, but i was wondering if any of you techies could answer this; Does using a vertical mouse require a different sensitivity? I usually use a relatively low sens with a dpi of 800, but i find it difficult to match this up in aim.pro.... any tips or full on answers would be greatly appreciated!

Speaking just of the difference from vertical mice to normal mice here. I'm assuming otherwise it's an identical mouse, for the purpose of this conversation. Different electronic components are different, obviously

You shouldn't require different sens, but you will have a greater ability to move at the wrist (some 300% greater motion, with a huge amount less stress, it's a big deal) which means better control of finer movement; basically you can run a higher sens if you want, without the potential loss of precision that would normally come of it. Or you can just run the same sens and get better precision.

You're going to have to train your brain that the 'left and right at the wrist' movement is now 'up and down at the wrist' (actually it's the opposite, I'm speaking from the perspective of a 'normal' mouse user) which is very easy - you just use the mouse - but takes some time as you are physically destroying mental pathways and creating new ones (you're also going to be reconditioning muscles and stuff but that's less relevant here). If you've ever been forced to use a mouse left-handed (if you've ever broken an arm you know what I mean) you know that it just suddenly 'clicks' after some time and then feels completely normal. Same will happen for changing to a vertical mouse. It's usually about 2 weeks. If you want it to be fast, don't swap from one to the other, force yourself to use the new one all the time.

TL;DR just give it time.