PAPI is still subject to erroneous indications in mist/moisture except that the colour is affected rather than the pattern. As some airline pilots are partially colour blind yet can still legally hold an airline pilot's licence, it becomes a serious matter when they cannot always interpret a normal PAPI because of difficulty in seeing the red lights. Imagine the problem if they see erroneous indications with colours merging in mist.
They have no problems with T-VASIS as it is not colour discriminatory -unless you are below 1.9 degrees when it goes all red - but you would have recognised a low approach before then. Unless the fog/mist bogey appears.
It is not that PAPI is more safe than T-VASIS. It is cheaper and easier to maintain, so I understand. That is why PAPI is appearing at more aerodromes in Australia and replacing T-VASIS.