Enter NCR and return to Stark

After completing Sergeant Stark's previous quest, you can ask him for another job. 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 there and enter. You can explore if you like, but all that's required is that you go in but once. Head back to Vault City and report in back to Stark.