Hi Paul,
First, you don't actually have to take the toilet apart to get to the EZ Slide valve, so you won't "already be in there" to take out the vinyl skirt. And doing it then won't save you any time, either, so unless you feel inclined to remove it anyway, you can procrastinate away.
Second, I just replaced by EZ Slide valve a few months ago because the old one had cracked. I can't remember if mine came with the donut or not, but perhaps you can take a pic and post it. I do remember it came with an o-ring that fits firmly around the chute part of the valve that goes through the donut.
I have replaced the donut separately, but the one I use is Thetford part #33364, as shown in this parts diagram:
http://www.thetford.com/HOME/PARTS/P...8/Default.aspx
They run about $5. You might consider just buying a one of those and compare it to the one you have. If they are the same, you are ready for the next time you pull the toilet -- it's not if, but when.
And yes, whenever I reinstall the toilet with a new seal, I have my lovely assistant site on the throne to compress it while I attach it to the floor. But -- DON'T SIT ON THE LID -- it will crack. Sit on the seat as normal.
Dave