I think its impossible to complete any of the FO games as a true pacifist
m8, are you a tourist? True pacifism without killing anybody IS what this franchise is known for. There were barely any games before, or since, that let you go thru the entire game without killing anybody, neither in the main quest or the side quests or even any forced random mob encounters. Fallout 1 made it possible, Fallout 2 made it possible, New Vegas made it possible. It's only the Bethesda entries that force combat and murder upon you to some extent, altho there is a mod for Fallout 3(made rather recently) that does make true pacifism possible
Is it easy? No, but it's certainly possible. Hell, one of the three archetypes the entire franchise build itself around for as it's foundation IS a pacifist that is supposed to talk the final boss to death, that's why you can make The Master kill himself thru dialogue(and why not being able to do so with Frank Horrigan in 2 was considered a big deal at the time) without even needing to kill anyone at the final dungeon either. In both cases, since Fallout 1 has two final dungeons. The diplomatic character is also the canon Vault Dweller that defeated The Master in Fallout 1 to drive the point home.
This is what 20 years under Bethesda's thumb does to a franchise, people barely even remember why it earned it's reputation in the first place, as an RPG with actual choices and not merely a hack n slash like everything else on the market, Fallout is what revolutionized CRPGs around that time. Not sure what's sadder, that people forgot about that or that more games don't offer complete pacifism options(or option to kill everyone) even today. Or maybe it's the mentally handicapped children that make up the mainstream Fallout fanbase who think it's supposed to be a silly comedy because it has talking mutants, literally what Emil said. That Creetosis video was really hard to watch, not even as a fan of the franchise but because I struggle to think how stupid these people really are.
I'm thinking of getting back into the game (New Vegas), but I wonder what sort of character archetype I wanna go for.
That depends, what base do you want for your character? Combat Boy, Stealth Boy, Diplomacy Boy, Science Boy ect.
I could recommend you weapon and armor builds, story for your character ect. but as I said above, depending on how you play you may not even need that many weapons unless you're going for a straight-forward Combat Boy run. I found that playing with the game's systems or trying to get thru the game in the most efficient way possible is more entertaining than just running around as a meathead with a backpack full of guns. At the end of the day, combat in both Gamebryo and the old isometric games is merely passable, not fun. Even Fallout 4 is merely mediocre and that game had a full combat overhaul to play more like a modern shooter.