Yes you can use a ZR-6 and VS-6 without the Niles keypads. I have done it twice already. The first one was 2 ZR-6s and 2 VS-6s are controlled by and XP-8 with RS232. The user can use one of the five T2-C floating remotes to choose any zone they are in, or some rooms have an RK3 also.
I am currently finishing a smaller install with one ZR-6 and one T2-B/RP-1 for control.