The Vikings have agreed in principle to a one-year contract with defensive end Lawrence Jackson, according to a person with direct knowledge of the deal. The move adds a situational pass rusher to the team's defensive line, where both Jared Allen and Brian Robison are coming off shoulder injuries. Jackson had 13 sacks his past three seasons with the Detroit Lions, posting his best year in 2010, when he recorded six sacks. He played the first two years of his career with the Seattle Seahawks, starting a combined 24 games and posting six sacks.