I have a different vendor's EFIS and a VP/X unit.
The trim monitoring is just reading the resistance of a potentiometer.
I would recommend taking your own reading with a VOM and both full and down trim positions.
I would then ask Dynon if there is a debug mode in which you can tell the value of the data they are receiving from the VP/X.
Chad should be able to tell you what the VP/X outputs based upon your resistance measurements. Then you should have enough data to tell where the problem may be.