Pancake box
So after much thinking, I decided to go with a pancake box. The vanity light I bought from Costco is just like any regular light fixture (unlike the strip light) and requires a box for the light fixture. But the pipe in the wall prevents a regular box if I wanted the light fixture to be centered with respect to the sink. The pancake box is 1/2" thick and is perfect. Bought some hangers so I can attach the whole thing to the studs. Image by Vincent Ma