What is the coordinate reference system of the Vision-RTK2?

The receiver output is always in the coordinate reference system (geodetic datum) used by the correction data service. In RTK mode, the receiver does relative positioning with respect to the base coordinates provided by the correction data service. The system in which those base coordinates are is entirely up to the correction data service provider. The receiver does not need or use this information at all. This is true for all ECEF (XYZ) output. For lat/long/height (LLH) output, the WGS84 parameters are used to transform ECEF XYZ to lat/lon/height. In non-RTK mode the output position is WGS84.