Tournament and League Rules For Dominion
There are intentionally no official tournament rules for Dominion.

There is a long, meandering discussion of tournament rules for Dominion on BGG.

I was pretty happy with the tournament rules used for Dominion at the 2010 World Boardgaming Championship.  Two things that it got right were


An emphasis on game placement rather than points per game to determine advancers
Playing two games per round and reversing seating order between games


But don't expect anything near consensus on the best tournament rules for Dominion.