NMEA Velocity Support for Skytrak S2525F8-RTK

I am trying to implement support for the $PSTI,030 message that the Skytrak S2525F8-RTK modules output. This message includes the East, North and Up velocities, I can decode the message just fine but am not sure quite how the GPS state should be updated with respect to which of the NMEA messages should set the velocity info and when. Currently when RMC and VTG messages are processed, a call to fill_3d_velocity() computes the N/E velocities from the course and speed information. The GGA message does not set these state variables. If this new STI message provides the 3 velocities at 5Hz is it sufficient to simply store the velocities in the current GPS state at this point and remove the calculated values from the RMC/VTG message processing?
Any help appreciated.