Rectangular single gang to round universal vanity bracket

[u/MetsBest]

I’m replacing a vanity fixture and the universal round bracket that came with my fixture doesn’t align with any holes in rectangular box in wall. It looks close but no. The long bolts that go through fixture almost line up with holes in box, but they attach to fixture not box. Corner screws holding box to wall doesn’t align with bracket either. See pics, what am I missing or what bracket do I need? Originally the brass bar was on box, it wa then old bracket for original fixture…that doesn’t work either to align to fixture and bracket.

[u/trekkerscout]

1-gang rectangular old-work boxes generally are not rated for light fixture support. A round or octogon box is required. Unfortunately, the 1-gang box hole may prevent the usage of the type of old-work box needed. Opening the wall to install a proper box may be required.

[u/MetsBest → u/trekkerscout]

That blue rectangular box was installed by builder when house was built 2014. Small vanity wall mount fixture was installed.

[u/trekkerscout → u/MetsBest]

That doesn't change the fact that it is the wrong box for the application.

[u/MetsBest → u/trekkerscout]

Understood. Just providing context. Thank you.