Ross Davidson went into a late and high tackle on Karleigh Osborne, leaving referee Andy Haines reaching for his red card as soon as Osborne hit the deck.
Vale’s fans were incensed by the decision and even more so when Osborne opened the scoring in the second minute of first-half stoppage time.
Their defence failed to clear Glenn Poole’s left-wing corner and the full-back smashed a 10-yard shot which flicked Joe Anyon’s fingers before entering the net.
Dean Glover’s side attempted to hit back despite their numerical disadvantage and were the better side in the second half until Marcus Bean doubled Brentford’s lead in the 67th minute.
Brentford then rubbed salt in Vale’s wounds in stoppage time when Charlie MacDonald blasted his eighth goal of the season after a tremendous run from substitute Adam Newton.