A measured Vidmar declared that his side had their "pants pulled down" by Macarthur in Saturday's 6-2 hammering at the Bulls.
City lie seventh after their defeat but have made a swathe of moves to strengthen over the last fortnight, bringing Ryan Teague, Daniel Arzani and Marcus Younis in on loan and adding Harrison Delbridge.
But Vidmar, whose side remain without Mathew Leckie and Andrew Nabbout due to injury, said the new additions wouldn't fix City's issues alone.
"We haven't had really much competition at all this season in a lot of positions and we need the competition because it's healthy," Vidmar said.
"Marcus only came in a couple of days ago and hasn't played since early December.
"Daniel had a little clean out of his knee, he's not played at all this season, so he's only just come back into training for the last 10 days.
"He's going to need a couple of weeks.
"Harrison's season finished on December 4, so it's well over a month without a game and he's been training for a number of weeks.
"They're going to need time and they're not just ready to go, unfortunately."
Teague got through 90 minutes against the Bulls after making the switch back to Australia from Belgian club Mechelen to strengthen his World Cup selection hopes.
But the midfielder would have done little to impress Socceroos boss Tony Popovic, who was a keen observer at Campbelltown Sports Stadium.
Younis looked confident and contributed a well-taken consolation goal after landing from Danish outfit Brondby earlier in the week.
"He is an explosive guy, he can run people down and get in behind the defence, and that's what he did," Vidmar said of Younis.
"He took his goal very well but he's also rusty because he's played very few minutes.
"They're all going to need a little bit of time but unfortunately, we don't have much time so we have to get our skates on for sure."
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