So I guess my answer was not clear enough.

Generally speaking, in Germany you can almost always use all regional trains (e.g., RB, RE, or S-Bahn) as well as busses, trams, metros/subways operating within a transit authority's area. It does not matter if the operator is National Express (as in case of the RRX), Trans Regio, DB Regio, KVB, HLB, a Wupsi bus, or whatever. Means of transportation that may be excluded are typically funiculars, specialty narrow-gauge railways, and ferries. Furthermore, long-distance buses (e.g., Flixbus, LH Express Bus) , high-speed trains (e.g., ICE, IC, EC, or ECE), other long-distance trains (e.g., Flixtrain), and similar are either excluded or require an upcharge.

So the answer is that you can most definitely use the RRX provided you have a valid VRS ticket for the zones you travel through. My previous post kind of suggested this to be the case, as the VRS search engine will suggest you take the RE1 when it suits the search query.