Ah the good old tamarind. I've become a fan of "TRS Tamarind Concentrate" for use (1 tsp) in my Vindaloos. This was what I was looking for in perfecting my Vindaloo in a long time.
I know most people here don't seem to be friends with a "sour" Vindaloo, but that's how I know it from my fav restaurants here (on the continent that is). Now I use 1 TBSP vinegar + 1 tsp tamarind concentrate (+ 1 tsp green cardamom crushed) and it' s all it needs to get that special bang!