You probably want to check into Trimble Positions. Make sure it supports your device. You might get a Trimble salesman to give you a demo with a device having ArcGIS Mobile and Trimble Positions. If you have an IP address, port number, and credentials for a certain VRS, you might want to have those ready for the salesman.
Some of the more advanced GPS receivers can output a VRS corrected NMEA message to ArcGIS Mobile. Of course the receiver has to be able to take in a VRS correction and work with those corrections. It is a bit of work to get this up and running.
If you are thinking of getting centimeter accuracy vertical positions, check if ArcGIS Mobile can correct based on a geoid model.
