Pro-Football-Reference.com has used Approximate Value "to come up with a post-merger all-franchise team for each franchise." Check out the NFC North.
I'm a bit surprised that Daunte Culpepper comes in ahead of Fran Tarkenton, and Matt Birk comes in ahead of Mick Tinglehoff.
According to this standard, the three greatest Vikings are three of the team's Hall of Famers: Alan Page, Ron Yary, and Carl Eller, who each have over 100. The other 90+ players are Cris Carter, Randall McDaniel, John Randle, and Chris Doleman.
If you really defined the Viking franchise's greatness over the years, you'd probably start at defensive line. There were the Purple People Eaters (Alan Page, Carl Eller, Jim Marshall, Gary Larsen), and later players like Chris Doleman, Keith Millard, John Randle, Henry Thomas, Pat Williams, Kevin Williams, and now Jared Allen.