City (30 points) go into Sunday's clash with Perth - who cannot make finals - four points behind sixth-placed Western Sydney Wanderers, who lost in a dramatic finish to Sydney FC on Saturday night.
That result lifted the Sky Blues (38) to fourth, level on points but ahead of fifth-placed Macarthur FC courtesy of matches won.
Realistically, Vidmar's side need to win at AAMI Park on Sunday to thrust themselves back into the top-six mix with two games to go.
Vidmar conceded City had shot themselves in the foot too many times throughout the season, but still had an opportunity to redeem themselves.
"We've got ourselves into a pretty tricky situation ... now we're in a situation where the margin for error is completely narrowed," he said.
"So it is a must-win for us.
"We've got to get to the point now where we can't keep going against us.
"We're all aware, everyone knows it's a must-win game.
"We've got to go out there, give our very, very best - and if we can give our very best, our form at home has been pretty good throughout the whole season.
"It gives us a good leg up."
City will be without winger Marco Tilio for at least two-to-three weeks through a quad strain while Scott Galloway remains sidelined and right-back Callum Talbot is away with the Olyroos.
Vidmar remained wary of Alen Stajcic's Perth, led by Socceroos striker Adam Taggart, despite their position at the foot of the table.
"They're dangerous, they're unpredictable, they've got guys like Tags and (Daniel) Bennie in the front that can really do some damage and a midfield that's pretty clever," he said.
"So it's going to be an absolutely tough game for us.
"But we know where we are in terms of what we need to do to get the result."
Sunday's match at AAMI Park will mark Vidmar's return to the touchline after missing three games through illness.
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