The Catamouts have won 15 out of 16 games.
Vermont, which defeated America East Conference regular-season champ Stony Brook in the finals of the league tournament to earn its NCAA berth, will next meet Midwest Region No. 1 seed North Carolina in Greensboro, N.C., Friday.
Lamar held a three-point lead with 12:48 to play in the first half, but managed only nine more points during the rest of the period and trailed at intermission 30-21. The Cardinals came no closer than seven the rest of the way.
The setback for Lamar quickly ended the initial appearance in the NCAA Tournament as a head coach for Pat Knight, son of former Army, Indiana and Texas Tech coach Bob Knight.
The younger Knight completed his first season at Lamar (23-12).
Matt Glass added 11 points and Sandro Carissimo had 10 for Vermont (24-11), which is in the NCAA Tournament for the second time in three years. Lamar was led by Devon Lamb with 16 points.