Striker Vydra, who is on loan from Udinese, came off the bench and slotted in what was his maiden Baggies goal in the 86th minute to steer the hosts to their first point in five Barclays Premier League games.
Reflecting on the strike and the result it secured, Downing - in charge on a matchday for the first time since the dismissal of Clarke and his assistant Kevin Keen - said: "I just think, with the difficult week we have had, it is a big boost for us.
"We have likened it to Gareth McAuley's late goal against Fulham (in a 1-1 draw in September) after we had made a poor start to the season.
"I think it was that sort of feeling in the dressing room. I wouldn't say we were jumping (for joy), but certainly it stopped the rot, and that was important to do."
West Brom looked short of confidence in a first half that was low on entertainment and saw them fall behind in the 28th minute when Jake Livermore, on loan at Hull from Tottenham, finished off a counter-attack.
But after leaving the field to the sound of boos from their own fans at half-time, Albion improved in the second half, with substitutes Saido Berahino, Vydra and Zoltan Gera - who teed up the equaliser - giving them fresh impetus.
And asked what he had expected before the contest, Downing said: "I envisaged that it was going to be a tight game, and I envisaged that the team would be short on confidence a little bit, with a little bit of edginess and tension to them.
"Obviously the crowd got like that as well in the first half, and rightly so, but I thought in the second half we got some sort of momentum to our game.
"I don't think we created a massive amount of chances, but we got some momentum, a lot of possession, worked the ball and showed perseverance.
"I know from working with these boys for four or five years that they have big hearts and that they really want to do well, and they kept going right to the end."
He added: "The day I took over, I was prepared to try to enjoy it, but also get a response off the players, and I feel I have done that.
"I would have liked three points, but under the circumstances, we can't be too greedy.
"The game fluctuated, we had to make decisions and thankfully they paid off."
As for his prospects of becoming a full-time boss, Downing said: "I don't think it will be here.
"I'm here to support. I'm just trying to pave the way for the new regime.
"I don't expect anything to be honest. I'm really just taking it on a daily basis.
"I'm conscious that the club is looking for a new manager.
"I'm planning for Christmas - we have the game against Tottenham (on Boxing Day) and then less than 48 hours later we play West Ham.
"So I'm just really conscious of trying to get that right."
Hull had a good chance to make it 2-0 in the second half but were denied by goalkeeper Ben Foster - who was making his first appearance since sustaining a stress fracture in his foot in August - as he pulled off a smart double-save to keep out a Tom Huddlestone effort and follow-up by Ahmed Elmohamady.
Tigers boss Steve Bruce admitted he had been left at the final whistle feeling like his side had been defeated.
"The dressing room is devastated that we didn't hold on," Bruce said.
"It (the goal) is the only time really in the game that Shane Long (who fed the ball to Gera) has got down the side of us.
"It feels as if we have been beaten.
"I think a big turning point in the game was the chance for Elmohamady, where Foster makes the double save.
"It could have been a different scenario, but there you have it - that is where we find ourselves, and we will move on."
West Brom remain 16th in the table, three points above the relegation zone, while Hull stay 12th.