Despite much of the early play being dominated by Vale, it was Rovers who threatened first when Rickie Lambert's low shot from 15 yards produced a comfortable save from Joe Anyon.
Bristol began to establish a foothold in the game after 15 minutes with David Pipe's deflected shot bouncing off the side netting and earning his side a corner.
Pipe came even closer six minutes later when a superb left-sided cross from Joe Jacobson found the winger lurking at the far post, but his poor header rocketed straight over the bar.
Jason Talbot came close to giving Vale the lead when, after a long spell of possession, he fired a 35-yard shot that forced a full stretch save from Steve Philips.
Luke Rodgers stretched his legs five minutes before the break when he collected David Rocastle's pass to beat two of the Bristol defence, but the final shot failed to hit the target.
Only 15 seconds into the second half, Bristol almost managed to end the stalemate with Richard Walker's flick setting up Lambert for the volley, but the ball fell wide of Anyon's right post.
The Valiants were given the opportunity they needed in the 65th minute when Phillips brought down substitute Calum Willock on the edge of his box to concede a penalty.
Rodgers had no problem stepping up to the spot and dispatched the ball straight down the middle with power.
A poor challenge by Rocastle provided Rovers with a free-kick in a promising position soon after, but Lambert's effort had a fraction too much height, dropping onto the roof of the net.
The Pirates stole themselves an equaliser in the 77th minute when debutant substitute Andy Williams found himself in the right place at the right home to score a tap-in from close range.