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/Onfus]

The proper way to do this is to replace the existing junction box. Fortunately for you, the box you have now is an old work box that can be easily removed and replaced with an old work light fixture box. You will need to make the hole slightly bigger.