Downing took over as caretaker boss at West Brom following Steve Clarke's dismissal a week ago, and was able to prevent a fifth straight defeat thanks to a late goal from substitute Vydra at The Hawthorns.
Jake Livermore's strike was the only talking point of a relatively dour first half, with the lively midfielder calmly finishing a one-on-one after 28 minutes.
Livermore looked to have registered the only goal of the game but Vydra rolled home an effort from inside the area to salvage a point for the hosts and deny Steve Bruce's men their second away league win of the season.
Downing recalled Ben Foster and Scott Sinclair from foot and hamstring injuries respectively while James Morrison earned a recall to the starting XI.
Boaz Myhill, Claudio Yacob and Morgan Amalfitano were the men who dropped to the bench.
Bruce's Hull side were unchanged from the 0-0 draw with Stoke City last time out as defender Alex Bruce shook off a groin problem to retain his place.
Following a quiet opening spell, which saw both sides cancel one another out in midfield, the visitors hit West Brom on the counter to take the lead.
Danny Graham retained possession well after breaking away and, when support arrived in the form of Livermore, he played in the Tottenham loanee, who slid the ball under the onrushing Foster.
David Meyler's deflected effort almost looped in to double the visitors' advantage six minutes later, while Sinclair came close to fashioning a clear chance, only to be denied by a last-ditch James Chester tackle.
That sparked a late period of pressure from West Brom but they were unable to avoid going in behind at the interval.
After Saido Berahino had replaced Sinclair at half-time, the home side started the second period brightly and Stephane Sessegnon stung the palms of Allan McGregor with a fierce effort.
McGregor was also equal to Gareth McAuley's header before Foster pulled off a double save to deny Tom Huddlestone and Ahmed Elmohamady.
Morrison fired a long-range effort over the bar after 72 minutes but, just when it appeared that West Brom were set for defeat, Vydra applied the finishing touch to a fine move, slotting home Zoltan Gera's throughball four minutes from time.
Despite seeing victory snatched away from them late on, Hull were able to extend their unbeaten run to three matches thanks to a third consecutive draw.
