Enter NCR and return to Stark.
After completing Sergeant Stark’s previous quest, you can ask him for another job. The sergeant received intel on the NCR expanding their military strength and this worries him. He tasks you with finding a route to NCR along with entering the town to scope it out. He also conveniently marks the location on your map.
Travel to the republic's capital and enter the town. You can explore it if you like, but all that's required is that you go in but once. Head back to Vault City and report back to Stark.