In case the GMPE has been calibrated for a specific vs30 value, like in this case, we usually do not define the vs30 as a site parameter. We report this in the documentation, but we leave the REQUIRES_SITES_PARAMETERS empty, that is:
    REQUIRES_SITES_PARAMETERS = set()

As an example see 'campbell_2003.py'. This allows for a more flexible usage of the GMPE.