What do I need to do to add stools to my vinyl windows?
Removing the sheetrock will lower the finished stool height a bit which will help maintain a more even reveal on the window, and  make it look less like the stool was added on.  Cut around the edges first with a utility knife to minimize damage to the surrounding jambs &amp; wall below and then just pry it up.  

If you really want to keep the same reveal as the old sheetrock, you could build up your new stool out of 1/2" thick stock (assuming the sheetrock was 1/2").  Make one piece that is the size and shape of the entire stool, then thicken it where it projects out from the wall by laminating another 1/2" thick piece underneath.