How to effectively engage enemies at medium to long range?

VATS considers distance in its hit chance and will frequently miss. Long range combat should be done without VATS.

Each weapon has a Spread rating. Spread is a measure of how far the bullets will deviate from the cross hairs. 0 Spread means that it shoots exactly where you point when shooting outside of VATS. Firearms skill affects damage done on hit, but does not affect spread.

Spread   Weapon
0.0      Named hunting rifles (best choice if you find one)
0.0      Sniper rifle
0.0      Laser rifle
0.0      Laser pistol
0.3      Hunting rifle
0.3      Scoped Magnum
0.2      Plasma rifle (slow projectiles = bad for long range)
1.5      Assault rifle
2.0      Minigun

Laser rifle is the goto weapon for accurate long range combat. Its main problem is low damage (slightly lower than a hunting rifle). Named hunting rifles are completely accurate and high damage. If you want to engage at long range, go get one!

It's my experience that shooting at long range is ineffective even when you have quite high firearms related skills.

As you say, the solution therefore is to close range. There are several tactics for doing this, including increasing your Stealth skill and crouching to approach (i.e. sneaking up on the enemies), Stealth Boys (expensive but effective), and use of terrain to remain in cover while advancing.

The "Gunslinger" (1 handed) and "Commando" (2 handed) perks will improve your VATS accuracy. "Sniper" (at level 12) will improve your accuracy with VATS headshots. These will push up your chances of hitting at long range.

Unlike some answers above stated, there is no bullet drop in fallout 3, or New Vegas. However the game does have issues with scopes being misaligned, which means at longer ranges, the centre of the scope may not be where the reticule is actually pointing

The Big Guns, and Small Guns skills, do actually affect both Accuracy (Spread), and Damage for guns effected by them. Energy weapons skill only increases Damage.

If you want a exhaustive list of all weapons, and there stats, including spread, click this link

Crouching also reduces spread, as does aiming down sight with the weapon. See this for more info.

The "accuracy related perks", only affect weapon accuracy in vats, and are multiplicative based on the number they state.

VATS should be avoided for long range combat, as distance reduces accuracy,meaning at any meaningful distance, your accuracy, even with VATS accuracy perks, will likely be very low. However, with certain guns, such as the alien atomizer (which has a very low vats shot cost) from Mothership Zeta, certain builds allow 10+ shots in vats with it, which with combined with the Concentrate fire perk, can raise your chance to hit by over 50%! Of note is the fact that the alien atomizer also has 0 spread outside of vats also.

The best advice I can give you, is to get a Sniper Rifle, or a Hunting Rifle (The Unique Ol' Painless is easy to acquire hunting rifle wise, boasting no spread!!!).