First, realize that when your "form and accuracy have taken a turn for the worse," this has nothing to do with the way you hold the string or whether the bow is vertical or not. If you were shooting better before, holding the string and bow however you held it, and started shooting worse, holding it the same way, then you must have changed something else.
When you switch to another method, like 3 under or holding the bow vertical, everything is new and you become more aware of what you're doing, for a while. I happen to be a big fan of Jimmy Blackmon, and try to do a lot of the same things he does. Maybe you're better off holding the bow vertical and holding the string 3 under. But that's not the reason you started shooting worse. Being aware of what you're doing during the shot, so you can recognize when you change things, is the skill you need to develop.
Start with a simple tab, just a piece of leather with a hole in it. I have tried a lot of them, and I currently like the tab sold by Rod Jenkins of Safarituff, but there are a lot of good ones out there, and the differences won't affect the way you shoot that much. My experience is that when you switch from split to 3 under you need to raise the nock point about 1/8".