Domdaniel: Actually, it's all known theory up to move 20 -- and Vaganian is one of the all-time greats with this 'retreat' line (5...Ba5 rather than the mainline 5...Bxc3+). Black often sacs the exchange in this type of position - sometimes with ...Rg8xNg5, sometimes in the manner of this game.
Admittedly there's a nice tactic in the zwischenzug 20.Nxd8 - Black obviously has to recapture. But, even with the extra exchange, White is already in trouble, and was lost within a few moves.
Black's central pawns, and the coordination of his pieces, mean that white gets no chance to consolidate his material advantage.
Black also wins quickly in a line like 21.Qf3 d3 22.Qxc6 d2+ 23.Bxd2 cxd2+ 24.Kf2 Bb7 etc - the white king is helpless whether or not black regains the exchange with ...Bxh1.
There are no real improvements for white after move 21 - so, pretty as the zwischenzug may be, it's actually a losing move.
This opening variation was (most recently) fashionable around 1998-2002, but it's dropped out of use a top levels recently -- I think this game was actually Vaganian's last significant win with it.
It has a long history, though. Botvinnik played it in the 1940s, a group of Swiss players such as Hug took it up in the 1970s, and the Armenians were at the forefront in the 1990s. It's been named after all of these at various times -- I call it the Swarm, short for <Swiss-Armenian> Variation.
The Swarm will return.