wo ping, you appear to be mixing up VASI with PAPI, not surprising as many installations are misidentified or ill defined e.g. “PAPI-VASI”. Each system provides a visual glidepath via different mechanisms. There are other sources of confusion, e.g. ‘T’ VASI, a good system found in Australia, but different again.
VASI, Visual Approach Slope Indicator, is the older system based on 2 sets of lights (red and white) set to the required glidepath angle. The boxes are located down the length of the runway, spaced either side of the GS origin (not to be confused with a second set of ‘long body’ PAPIs at the larger airports). Due to the type of light and installation, the VASI beam width is relatively wide resulting in a diffuse red/white mix near the change over point, i.e. ‘pink’ appearance. This and the deteriorating beam accuracy at low altitude (relatively large beam width) results in VASI being of little use below 200ft, i.e. they are not reliable.
PAPI, Precision Approach Path Indicator, is based on a set of 4 lamps located at right angles to the runway usually just beyond the GS origin. Each lamp unit provides both a red and white section of the beam. The changeover between the two is ‘sharp’ as the lamp unit uses a focussing lens and colour filter (like a projector) giving each a narrow beam width; this eliminates any pink zone and in theory enables the system to be used at much lower altitudes. Small differences in the beam setting either side of the required glidepath provide the 2 red / white etc combinations. A limiting factor is usually the narrow beam width at low altitude (small vertical displacement for colour change), but PAPI is still reliable although not very easy to follow.
Atreyu, to achieve a ‘slightly’ below glidepath position at 50 ft implies an earlier manoeuvre. In addition to any ‘unstabilising’ effect (either by reducing power or nose down pitch, both ill advised at low altitude), the obstacle clearance is reduced. IIRC the lights on the bridge just short of rwy10 are quite close – then you could consider the height of a London double-decker bus (which I believe was considered in the hazard assessment).