Truth be told, I've never known how to play a high Charisma character who was not also strong in Intelligence. Intelligence often makes a huge difference in available dialogue options.
(Certainly make it exceed a 3. 9 to start might be better.)
If you intend to end the game without combat, A very high Speech is necessary. In Fallout 1, Science or Medicine at similar values is absolutely necessary as well. A high Science skill is always fun anyway.
If that's not as important to you, don't worry as much about the Intelligence. That's necessary for achieving dialogue options which alter the flow of the game, but not too important if you're just trying to avoid a combat or two and be well-liked.
Since you're already looking at one high stat outside the combat set, you might consider building a character who is simply ill-suited to combat in general. This would mean getting by on wit, charm, and a high Outdoorsmanship skill.
Alternatively, Small Arms in Fallout 1 (and, to a lesser extent, 2) can allow you to snipe from a fair distance to handle most of your problems. That works better without teammates.
I suppose I can see worse ideas that being dashing and pretty, strong (6 at least) and relying on Big Guns. They tend to solve a lot of problems, too.
However, I would recommend Sneak if you want a problem-solving TAG skill to get around your problems. There's other options, but I really feel avoiding unnecessary combats is very important to a charm build, and a lot of people simply aren't in a talking mood! (Note: Outdoorsman is very, very important to this as well.)
If you wish to rely on equipment so as to win unavoidable fights, I recommend either Gambling or Bartering be brought to or near 100%. Gambling in particular you could probably get by with a bit less. I did okay with 70%. Both become unnecessary when your equipment is maxed out, so I wouldn't waste a TAG skill on either, though there might be some role-playing fun in being just that good a barterer. It could be a fun way to learn the maximum number of coins or caps you can carry in one place, and it does mean you get to solve your problems with money. That's oddly satisfying in my experience.
So I'd say:
(Option 1) Raise your intelligence as well. Get your Science or Doctor skills very high. Both is unnecessary in F1, though in F2 Doctor opens a few interesting doors if brought rather high.
(Option 2) Sneak, predict, and solve your problems in roundabout ways, and rely on charm and wit for situations that aren't game-ending in importance. Outdoorsmanship, Sneak, and a solid Perception.
(Option 3) Pick one combat skill to back up enough to solve your problems, but don't build your strategy or whole Perk list around it; just enough to solve intractable problems. Recommend Big Guns and Strength if you go this route. Not efficient, but hilarious and oddly versatile. More practical, of course? Sniping. But just not as hilarious.
(Option 4) Rely on equipment for combat problems, letting that make up for your failings in most stats. Throw your stat points into Charisma, Intelligence, Perception and maybe luck, and equip your (several) buddies very well if you're in Fallout 2. Enjoy having more money than Pluto.