In the process, City ended Sydney's season, with Adelaide United, whose players were watching on from home, to take the sixth spot in the finals and face third-placed Western United in an away elimination final.
The Sky Blues, with their season on the line and only needing a point to make finals, turned in a horror defensive showing.
Marco Tilio turned on a masterclass as City raced a 3-0 lead in just 34 minutes and never looked back.
City have overcome a horror run with injuries to hit form at the right time and now get a week off before their two-legged semi-final.
"It was a great performance," Vidmar said.
"I think the mindset, our mentality was perfect tonight, especially the way we started.
"We really controlled the game early and sort of set the tone.
"Really pleased for the boys. They've gone through some challenges this year. The whole group has, to be honest.
"Guys have stepped in and done a great job and we've got very good leaders here that have kept things together in the difficult moments. So, really pleasing.
"And certainly I feel that we got what we deserved through the course of the year.
"So, great to get second spot. Another trip to Asia for the club, which is brilliant, and a week's rest and we get ready for the semi-final."
Former Adelaide coach Vidmar joked he might hear from friend and outgoing Reds boss Carl Veart after keeping their season alive.
The hosts took the lead when Sydney goalkeeper Harrison Devenish-Meares failed to fully clear a Tilio corner and it fell to an unmarked Aziz Behich.
The City skipper turned onto his left then let loose a skidding shot that beat Devenish-Meares at his near post.
City doubled their lead in the 26th when Tilio threaded a tantalising long ball into the path of Max Caputo, who got to it ahead of Alexandar Popovic, then beat Devenish-Meares at the near post.
Eight minutes later, Nathaniel Atkinson burst forward and found Tilio, who brushed aside Popovic to score.
Devenish-Meares was forced off through injury in the 52nd minute, with departing club great Andrew Redmayne replacing him.
But just six minutes later, Redmayne too watched the ball hit the back of the net as Andreas Kuen breezed past Popovic before rifling home.
Patryk Klimala headed in a Douglas Costa cross in the 66th minute in a rare bright moment for the visitors.
Medin Memeti lashed home in added time to complete the rout.
It leaves Sydney to confront the post-mortem of their season.
"It was disappointing," coach Ufuk Talay said.
"All we needed was one point from the last two games to get ourselves to play finals football and, unfortunately, we didn't do it.
"And if you look at the performance tonight, I don't think it was good enough to play finals football."
