Promotion-chasing Brighton, with Tuesday's trip to Burnley in mind, made seven changes to the team beaten at Derby last weekend.
And the surface water caused by a pre-match thunderstorm will also have been to Vale's liking with the sodden pitch another leveller.
Much of the early play was understandably bogged down in midfield and a couple of long range efforts by Brighton's Jon Obika and Rohan Ince proved harmless.
Andrea Orlandi went closer when his shot on the turn veered just wide of Chris Neal's right-hand post with the visitors gradually turning the screw.
Neal then had to get down smartly to keep out Solomon March's powerful shot before Brighton broke the deadlock in the 27th minute.
Orlandi delivered an outswinging corner from the left and Ince cushioned a classy left-foot finish high inside Neal's left-hand post from 12 yards.
The goal roused Vale into action and Tom Pope almost got on the end of Jennison Myrie-Williams' cross before big centre-back Chris Robertson headed a 36th-minute equaliser.
Chris Lines delivered a quality corner from the right and Robertson timed his run to perfection to bury a 12-yard header low inside Peter Brezovan's right-hand post.
Vale grew in confidence as an entertaining cup tie came to the boil, but were rocked back on their heels when Brighton regained the lead in fortuitous fashion two minutes before the interval.
Orlandi rolled a short right-wing corner towards March and the midfielder's intended right-foot cross looped over everyone and the ball dropped inside the stranded Neal's right-hand post.
Vale boss Micky Adams, who had two spells in charge at Brighton, replaced left-back Rob Taylor with Adam Yates at half-time.
But the visitors went for the jugular as Neal saved well from March before Orlandi had a goal-bound shot charged down.
Jake Caskey's fierce drive then forced an even better stop from Neal with Vale working hard just to stay in the tie.
Kemy Agustien was next to go close for Brighton when his 64th-minute shot crashed against the overworked Vale goalkeeper's right-hand post.
Adams brought on strikers Ben Williamson and Jordan Hugill to chase the game, but the visitors countered with a deserved third goal 12 minutes from time.
Ince's first-time ball sent Jon Obika sprinting clear of the Vale defence and the on-loan striker from Tottenham rounded Neal and rolled a left foot shot into an empty net from 12 yards.
Battling Vale refused to lie down as Hugill and former Brighton midfielder Doug Loft were denied close to goal, but Brighton stayed strong to see out the tie.