A grouped vector brush definition object should yield a vector result if a) all shapes only have simple flat fills and or flat transparencies, and b) the vector path does not contain sharp corners or cross it's self, or have too much of a curve.

I haven't got the time right now to figure out how much of a curve it takes but I don't think it's very good this way. I have submitted this to Xara support but I'm not holding my breath as TPTB at Magix are the ones that would need to be persuaded.