American 1/4" npt is 18 tpi, 1/4" bsp is 19 tpi.
American npt tapers at a rate of 3/4 of an inch per foot (1 in 16). BSP is a parallel thread also referred to as British Gas, British Pipe Parallel or Parallel Fastening thread. Various Symbols: BSP, BSPP, BSSPI, BSPF, BSPG, PS, R, G. British Standard Pipe - Taper -BSPT also referred to as British Standard Taper Pipe or Pipe Taper, or Conical Thread. Also Taper is 3/4" taper per foot ( 1 in 16 on the diameter) Various Symbols: BSPT, BSPTr, PT, KR, Rc.
Bottom line I don't think I would try and fit an American 1/4" npt fitting in to a Vincent tank unless I either made something to adapt the threads (which I've considered) or do a some surgery on the tank and fit an appropriate tapered bung.
Steven