On the 320, our procedure is to follow the Glide slope where there is one available in precedence to the PAPI/VASIS because they often do not coincide. Within our network this is easy enough as there are no glide slopes that have a TCH less than 50 feet.
In relation to a VASIS (albeit quite different to a PAPI) our wheel crossing height is 25 feet for a full flap landing with an eye height of 50 feet.
There are some VASIS' in our network that would have our wheels crossing at less than this 20 feet minimum, which, understandably, the company is not happy with. Therefore with said VASIS we will fly with one dot fly down or one dot high, indication and brief accordingly.
Due to the inaccuracies of visual approach path indicators we are instructed that their use below 200 feet is not recommended
What I have found to be a more useful in the late stages of approach, assuming reasonably flat ground on the approach to touchdown. I listen for "One hundred" (RA callout) and look where you're subsequently positioned, you'll start to get a good sense of when the "fifty feet" is going to be called out early or late. Ideally you'll hear it precisely as you passover the piano keys whether you can see them or not, you will know where they are.