I took this off of my clasified listing of the Compter Pen kit.
The Vista kit uses the same tube as the standard Sierra. The Vista is just a little fatter. So you can use the standard Sierra bushing if you are careful and check the size of the kit parts.
Bushing measurements
I.D. of both the standard and vista is the same .389
O.D of booth of the standard Sierra are .477
O.D. of the Vista are NIB .523 and clip .550
Bruce