Vasco, Goa, stubbornly defended a first half lead to shock Mohun Bagan 1-0 in a ninth National Football League match in Kolkata on Friday.
The visitors scored the all-important goal in the 25th minute through Alex Ambrose and then staved off a sustained Bagan onslaught to earn their first victory in the tournament and boost their points tally to five from nine matches.
Alex Ambrose's goal in a mediocre match at the Salt Lake stadium was, however, controversial, with the Bagan defenders claiming that Denis Cabral, who provided the final pass to Ambrose, was off-side.
Cabral, who received the ball from Menino Fernandes, set up Ambrose with a short pass and the latter re-directed the ball into the net on the run, sparking off protests from the local team.
Bagan, who have 11 points from 10 games, enjoyed the lion's share of ball possession and maintained overall supremacy for most part of the match but failed to restore parity despite getting some good opporutnities.
Vasco goalkeeper Virender Singh, who was adjudged man of the match, proved a big stumbling block for the locals as he kept his cool to thwart several Bagan offensives.
The woodwork also stood in the way of a Bagan goal in the 76th minute when Brazilian Beto's back-header following a Rennedy Singh corner crashed into the post and rebounded into play.