With the Vikings’ turnover in free agency and a slew of rookies added, more than a handful of positions are up for grabs on offense, defense and special teams. Some clarity was gained during the offseason program, such as Marshall Koehn’s legitimate chance to unseat Kai Forbath at kicker, but there’s much to sort out for the Vikings in training camp. Center: Third-year center Nick Easton held down the middle of the offensive line during much of spring practices and has led the competition with rookie third-round pick Pat Elflein, whom the Vikings view fits best at center after one season there with Ohio State.