It does seem a bit strange but if you look at the specifications for the VVRC 20cc engine at the VV web site, it shows a 15x 8 as one of the props suitable for the engine. It also shows an rpm range up to 9,000 rpm which I would guess a 15x 8 prop would approach.
I have always used a 17x 6 prop for my 20cc engines and they give about 8,400 rpm give or take a 100 rpm. I have both VV and DLE 20's.
Give a 17x 6 a try. If you get 8,200 to 8,500 rpm with that prop I think you will be very pleased with the engine performance.
Indiomike