the group of friends i play with have completed all the quests from the core game and WoD. i was wondering if it is necessary to purchase AoD before getting RtL?

I haven't played RtL, but it's advertised as being usable even if you have no other expansions and I haven't noticed any complaints to the contrary, so I assume you're safe.

You certainly don't need AoD. I've started a RTL game just recently with Vanilla Descent and I've been looking throughout the forums a bit trying to get a feel for the game.

Before I go any further, I'd suggest you go to the Support page for Descent and download the instruction booklets for the expansions. This will give you a much better idea of how these will affect your game, with the exception of the new cards themselves, since they aren't viewable in an digital format.

A couple of things about using Vanilla Descent to keep in mind:

How many Monsters are in a category

Vanilla
Humanoid: Beastmen, Ogre's, Troll's
Arcane: Skeletons, Sorcerer's, Demon's
Beast: Manticore,, Razorwing's, Dragon's, Naga's, Bane Spiders, Hellhounds.

Bane Spiders get ripped on a lot in the forums. They seem to be gimmick's without some serious upgrades and their large base works hard against placing such big and fragile creatures, especially as the dungeons in RTL are small. Hellhound's are almost worthless to start, but seem to pick up a little as beasts are upgraded. Ogre's and Troll's are still disparaged for their slow movement and melee only attacks, making the humanoid category seem week to a lot of people.

Well of Darkness
Humanoid: Kobold's, Ferrox
Arcane: Golem's

From what I can recall, Kobold's and Golem's aren't very strong in RTL

Altar of Despair
Humanoid: Deep Elves, Troll's
Arcane: Dark Priest's, Chaos Beast's
Beast: Blood Ape's

I gather that Chaos Beast's aren't worth much and Deep Elves are a mixed bag, probably dependent on play style. I've seen their pierce lauded for hurting tanks in the same sentence admonishing one for trying to take out a heavily armored tank.

Treachery

Treachery is a tacked on mechanic with just Vanilla Descent and I have no idea if it's really worth that much without the real cards for it. It seems much more necessary in LT encounters, though. I also don't think an overlord can scoff at increasing his starting hand by up to 5 cards in a dungeon. That's almost 3 additional turns off the OL's deck.

Hero's, Equipment, and Skills.

This is where I believe running a Vanilla descent game for the first time is a good idea. I've gathered that the most broken combo's (Broken still exists, though) for heroes are going to come from. Without the expanded pieces, it should be possible to have a smoother game, which could keep the game from unbalancing as badly. Also, I hear a bit about the Heroes really steamrolling things in Silver and Gold and I suspect that this will hold true even without the expanded equipment.

Finishing Up

So Vanilla Descent should be just fine with RTL and may be a better way to learn how to play the campaign. It looks at least as fun as the base game. My group highly enjoyed our first session and I believe that we will continue to enjoy this campaign, future campaigns, and the add on's as we purchase them.

Don't forget that RTL makes the game swing heavily towards the side with the best tactics in the Bronze stage. Be sure to give your first campaign a while to smooth out and get advice for both the OL and the players if you need to.