Twin Jock, the mechanism of VASIS result in ‘diffuse’ light at low altitude whereby the red/white beams become pink; thus they are considered to be inaccurate below 200 ft.
PAPI is a projected light system using a lens to focus the red/white light beams. Also, there is a sharp transition between the colours so they are usable at much lower altitudes. Some of the limiting factors involve the narrowness of the individual beams and the angular difference between the box settings, but in theory PAPI can be used to touchdown.
A widespread confusion in the industry is the interchange of terms where PAPI is often called VASI or a PAPI/VASI installation.