Virginia, led by the second-half scoring of Jeff Lamp and Othell Wilson, pulled out a 78-74 victory tonight over Louisiana State to salvage third place in the NCAA tournament.

The game began on a somber note when the public address announcer asked the crowd to observe a moment of silence for President Reagan, shot and wounded hours earlier in Washington.

Moreover, Virginia Coach Terry Holland and LSU's Dale Brown said it would be difficult for their teams to show enthusiasm tonight after their NCAA championship hopes were ruined in semifinal games Saturday. They were right. It was a sloppily played, foul-marred contest and both teams played, until the final five minutes, as if they would have preferred to be elsewhere.

Lamp scored a game-high 25 points, making all 11 of his free throws. Fellow senior Lee Raker also finished his college career on a high note, scoring 21 points.

LSU's Howard Carter fouled out with 7:33 left. Carter had 14 points and kept LSU in the game early.

Virginia all-America Ralph Sampson and LSU's top player, Durand Macklin, were among those who appeared to be going through the motions. Macklin didn't score, missing all three of his shots, and had five rebounds before fouling out with 12:12 to play. Sampson took only eight shots and had 10 points and 11 rebounds before fouling out with 9:30 to go.

When Sampson left, Virginia (29-4), which concluded its finest season in 64 years, led, 58-48. But the Tigers (31-5) began rebounding better and scoring inside. Leonard Mitchell (16 points) and Greg Cook (21) accounted for 16 of their team's final 26 points as the Tigers rallied to go ahead with 4:07 left.

LSU outscored Virginia, 19-8, over a five-minute span mainly because the Cavaliers made only three shots in the last 15 minutes. LSU went up, 67-66, when a long jump shot by Johnny Jones gave the Tigers' their first lead since Carter's opening basket.

But Lamp, who finished his illustrious career with more than 2,400 points, got a three-point play to put his team back up, 69-67, with 4:04 left.

LSU closed again, to 73-72, on a layup by Cook with 1:26 left. But the Tigers, who shot only 47 percent, had little choice but to foul and Wilson, one of his team's top free throw shooters, sank both of his one-and-one foul shots for a 75-72 lead with 1:17 left.

The Tigers' Jones scored again from long range to make the score 75-74 with 53 seconds left. Wilson was fouled again and made one of two free throws for an important two-point lead with 13 seconds to go.

LSU rushed downcourt, but reserve Tyrone Black's 18-footer from the corner had no chance. Raker came down with the rebound and closed out the game with two shots from the foul line.