Unless you are ex military or a black belt in some form of martial art you won't make it with ur plan
Why? Powder Gangers don't attack unless I piss them off, if I get stopped by them I give them what caps I have and move on.
After that I need to climb the cliffs east of Hidden Valley, which can be done realistically but can't in the game as there are invisible walls.
After that the road is safe towards Vegas.
Also legion would just enslave you in the game it REALY makes no sence why legion or NCR just let you help randomely
No, they wouldn't, do you even know anything about Legion?
They don't enslave Dale or other civilians in their lands and they don't enslave the Courier, so I don't see why I'd be enslaved if I go straight towards them and peacefully initiate relations.
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Also it makes perfect sense why NCR and Legion accepts help from a third party.
NCR is often restricted by politicians and military orders, if they can get something done by using a merc they will.
Legion has a war going on and they aren't exactly liked in the Mojave, so using a mercenary to get crap done on the d-low makes sense.
They don't just let you help randomly, they generally have a reason for accepting your help.