The threads on a Vortex Viper are 52mm. The objective lenses on numerous scopes are 50mm, however their housings are not always uniform in size or diameter.
If you go to B&H Photo or any other camera site/store, you can buy step up or step down adapters that allow photographers to use different filters on larger or smaller diameter lenses. These also work for scopes.
I mounted a Leupold Alumina 50mm cap on my Viper PST using a 52mm to 55mm adapter. Threading the Alumina cap onto the 55mm end will be a little bit snug, but it works.
As a sidenote, using camera filters on your objective lens (52mm filters for the Viper PST) will give you a sacrificial lens and a variety of filtering options for increasing intensity, plain UV filtering, etc. Higher end filters will prevent any distortion or issues with clarity or light transmission.
Hope that helps.