Derek Riordan levelled on the stroke of half time but, in a second 45 minutes packed full of goal-scoring chances for both sides, Vernon was the only man to find the net.
Obua passed up an excellent chance to put the ball in the net when his attempted tap-in was blocked by Fraser, but the Hearts man had already been flagged offside shortly before Suso Santana's volley was saved by the Motherwell keeper.
Basso came well off his line to deal with Danny Butterfield's hopeful ball into the box but McCombe tried to head it clear, leaving his keeper in no man's land as Watson directed a superb looping header into the unguarded net.