Yes, They're Compatible
Yes, either the VG400 or the RD6000 can act as a buddy-box to the other... as long as servo reversing, travel etc match. Oh yes, the alarms(countdown, low battery etc) on the RD6000 will only work if it is the master TX.
You can also get a trainer cord from (I think) Expert Electronics that will work with those radios... runs about $12.00 Canadian at the LHS.