As a result i use Trail Running, but it seems that as a result the VO2MAX is skewed. Running orienteering is in general much harder that trail running since you run off the trail directly in the woods.
I do not think that the name of the discipline you choose has any impact on the VO₂Max. The watch calculates the VO₂ on the basis of your HR data, and (if available) the oximetry data. It means the bigger effort you do, the bigger change of HR (resp. the oximetry too) will be detected, regardless what the activity is named.
If for some reason, you do not want to use the built-in Trial Run for orienteering, you always have the possibility to create your own custom activity and name it Orienteering.