Agility and Perception don’t help a sniper as much as you’d think in vanilla, beyond being prerequisites for some perks you want. IQ is highly valuable to anyone for skill points, and Luck is particularly valuable to a sniper because you’re going to favor weapons that tend to reward critical hits, and once you’ve lost surprise your chance of a critical depends on the weapon and your Luck stat.
Strength matters to a sniper in New Vegas, which wasn’t true in Fallout 3. More powerful firearms require higher STR to use. You’ll probably want at least STR 6 sooner or later, though you can start with STR 4 and plan on improving that via perks and bobbleheads.
The Speech skill has changed considerably. Instead of a die roll, all dialog options that involve a Speech check are pass / fail based on your Speech skill. Speech isn’t the only skill checked in dialog either, you’ll end up taking some points in otherwise low-value skills like Barter to pass checks. There’s an early game quest that introduces you to this idea. Ask around, and you’ll find out where you need the skills to pass checks. It’s well worth doing so, your odds of success go up considerably if you pass some or all of those dialog skill checks.
Beyond that, you pretty much must concentrate on increasing your Guns skill early. Once you can survive a basic firefight, of which there will be a lot, you can look into things like stealth. Remember that range by itself can give you surprise outdoors, even if your stealth skill is terrible.
VATS is of limited use to a sniper, since the accuracy falls off rapidly with range. You’re better off manually aiming your shots if the target is distant enough to justify a scope.
On the other hand, VATS is really, really handy for playing on the 360 once enemies are closer. For me, at least, I find it hard to aim quickly with a controller, and VATS improves things several different ways: time stops to give you time to react, you can select targets without having to aim, and there’s no chance they’re going to dodge my slow joystick aim, though the game rolls a die that you can miss shots that might otherwise be easy.
For a PC player, I’d say play it vanilla first. Vanilla is still a good game, and once you’ve finished the game on vanilla, you’ll have some idea which mods will change things you want changed. Mods can screw things up enough so some things don’t work as intended. When I played through heavily modded, I found one combat changed significantly because it made the Fat Man launcher difficult to use, and the encounter is scripted with the assumption that you’ll use one. There’s a Rube Golberg device in other location that will kill you if you’re playing with a mod that improves the radius and strength of explosions.
