I would probably do one of two things:
-extend you range a little bit ad get a Bushnell Elite 4200 2.5-10x50 with Firefly reticle
-forget about the whole 50mm thing (it is not all that critical) and get a Bushnell Elite 4200 2.5-10x40 with Firefly reticle
Vortex Viper 3.5-10x50 is a very nice scope, but the available reticles are a bit thin. In fact, most scopes in you price range have reticles that are, in my opinion, too thin for low light use. One possible exception is Leupold VX-II line which has a 3-9x50 scope with a thick duplex reticle. The problem is that I had more problems with VX-II scopes than with all other Leupold scopes put together. Coupled with rather emdiocre optics, I generally do not recommend VX-II scopes.