UNDATED - The Northstar Commuter Rail and the Northstar Bus Link will be running during Vikings games.

During all home pre-season and regular season games they'll have special trains and buses to-and-from Mall of America Field at the Metrodome.

You can get a Northstar Roundtrip Family Pass for two adults and three kids for $20 or less.  The bus will cost between $1 and $1.50 one way, which is paid for separately from the train ride.  Once you arrive at Target Field, you can transfer at no charge to the Hiawatha Light Rail line, which will take you to the game.

Resources
Metro Transit Website