Can't we omit "in" in the second case and say "Do you have this belt in black or green"?
I'm not a native English speaker but for me that sounds fine and actually better than having "in" there twice. Someone correct me if I'm wrong.
I am a native speaker, and that is exactly how I would ask it - "Do you have this belt in black or green?" Likewise "Do you have this shirt in medium or small?"
Why here is TAI instead of VAI?
With 'tai' the expected answers are "Yes, both" "Yes, green" "Yes, black" or "No". With 'vai' the only expected answers would be "green" or "black"