Its said that a neutral character will be very difficult to play as, so I can imagine a lot grinding until you are strong enough to blast your way in. :flamethrower:
I imagine if you work for neither faction there will be a different option. Such as this conversation:
NCR Ranger: "Hold up. I haven't seen you around who do you work for?"
PC: "[Lie] [Speech] I'm a trader. I was hopin' I could get through to sell my goods."
NCR Ranger: "[Success] Alright, but there is nothing ahead, but a bunch of Powder Gangers, and Deathclaws."
Or maybe some other skilled dialogue option such as [Guns]. Saying "Let me in or I'll blow you [censored] head off."