Check the PAPI to ILS GS alignment from the chart, the standardised (ICAO) arrangement requires them to be accurately aligned for normal sized aircraft. If not then the difference and conditions of use should be stated on the runway/approach chart e.g. two sets of PAPI, one for long body aircraft, or terrain/obstacle clearance.
All ILS GS bend upwards (flatten out) the closer you get to the ground – parabolic effect due to the reflected beam. Thus do not use the GS below the approved minima; obviously Cat 2/3 installations are better than Cat 1.
A true PAPI system uses a projected light beam which is very accurate, significantly better than VASI. Again check the charts re PAPI or a hybrid PAPI/VASI installation. The old style VASI was most unreliable below 200ft – ‘all pink’.
When visual, follow the visual cues but use the PAPI for additional information i.e. sudden change in vertical speed or floating high.