In an effort to get the pad to stop vibrating, you might have been pushing too hard.
You did the right thing in getting the correct pad/backing plates.
There are a lot of different variables on what can go wrong. Take a look around, as there are several instructional PC videos that are helpful.
I know there is a board member here that shows how to use the
Menzerna/PC on a black Tahoe/Suburban and
Meguiar's Online also has videos that are helpful.