Yes the SCX21 will work with both NAVNET VX2 and NAVPILOT 500.
Please note that with the NAVPILOT 500 you'll need to go to the Dockside Setup, and change the Compass Type to "Other" and set the Baud Rate to: "4800".
The odds are good if you have the DFF1 or DFF3 but not for the BBFF1 or BBFF3. We can't be 100% certain because the age of the Vx2 it might not pass the heaving information over the network to reach the sounder. I think it will. The DFF1 has heaving support but not entirely sure that the Vx2 will pass it. Regardless the SAT compass makes for a better pilot, better radar overlay. You don't get the radar skewing that you normally get with flux gates.