and I wonder why the T VASIS system ever went out of favour.
While T-VASIS is considered by many as far superior to PAPI because it does not rely on colour discrimination (the advantage being to pilots who may be mildly colour blind will have no problems interpreting T-VASIS light sources), nevertheless T-VASIS has more lamps and boxes and more expensive to maintain.
Both T-VASIS and PAPI are subject to certain limitations (erroneous indications) in misty weather, one of the important safety aspects of T-VASIS is that in a gross undershoot situation at 1.9 degrees the lights go red.
With PAPI, the presence of all red lights means you could be slightly under the ideal glide path to where all lights go red but that proceeding lower still below the glide path the red lights stay as is - in other words no graduated below all reds slope. Put another way, with T-VASIS all lights red means you are in a dangerous undershooting situation. Papi all red could mean anywhere between a mild undershoot to scraping the terrain.