Cook the onion down in a small amount of butter to very browned for a solid half hour at least.
Add in the garlic and cook 3-5 minutes.
Add in the curry powder and cook 2 minutes.
Add in the vinegar to deglaze the pan. Scrape well to get any browned bits off the bottom of the pan.
Cook 5 minutes to reduce and cook out the vinegar a bit.
Scoop into a blender or food processor with the sour cream and mayo. Blend to smooth.
For the sandwich:
In a decent amount of oil, fry some small white beans for about 2 minutes, then add in cauliflower and brussels sprouts.
Cook until everything is nicely browned but still has some crunch on the inside.
Spread some bread with the vindaloo sauce and load it up with the beans, sprouts, and cauliflower.