I want to use control system like VK to send the command through RS232 of VP1421 to the monitor to control power on/off TV.

Can VP1421 support to pass RS232 over HDBaseT?

Yes, it could, but the RS232 has to be connected to VP1421 display RS232 port.  Below is the testing scenario to explain:

1. PC or VK--->RS232--->VP1421 RS232 in--->VP1421 HDBaseT out--->VE802R--->RS232 cable--->VS0201

Command: sw i01 or sw i02 to switch port of VS0201

Result:

The command will only send to VP1421 to switch the input port of VP1421, not VS0201.

It means that the RS-232 is only for controlling VP1421.

2. PC or VK--->RS232--->VP1421 display RS232--->VP1421 HDBaseT out--->VE802R--->RS232 cable--->VS0201

Command: sw i01 or sw i02 to switch port of VS0201

Result:

VS0201 could switch port successfully.