The one-out blast from Valdespin put the Mets in front for the first time all night and gave them their second win in three games.
John Wall (0-1), called up this week from Class AAA Albuquerque, came on to start the 10th as the fifth Los Angeles pitcher of the game and he managed to retire only one batter.
Valdespin followed with his homer to right on a 2-1 pitch.
That gave the win to Bobby Parnell (1-0), who worked the top of the 10th.
The Dodgers' Brandon League blew a save opportunity in the ninth, giving up a two-out, RBI single to David Wright that tied the game.
Matt Kemp's two-run homer in the sixth had given the Dodgers a 3-1.