I have mine in part shade, average to dry soil, and find it blooms beautifully with almost no care. I suspect yours might be getting too much sun. I seldom fertilize my plants. Is it possible your plant is getting too much fertilizer, especially lawn (grass) fertilizer which is high in nitrogen?

I had about six plants of this Geranium in a garden (east-facing) that received full sun 'til about mid-afternoon.

I wonder if yours are too wet? According to the RHS 'The Plantsman', this "is a species of the cold alpine herb fields of the Caucasus where it survives on a Spartan diet among the rocks and cliffs."

Sorry, my RHS quote above was referring to G. renardii, which of course was one of the parents for 'Philippe Vapelle'. I still believe it applies to the cultural preferences of this plant.

'Terre Franche' is another nice cultivar (hybrid) of renardii. It is very similar to 'Philippe Vapelle'.
I forgot to mention that I grow both plants on a slope, in part shade. They need very little supplemental watering even in a dry summer, at least in my garden.

Here is a link that might be useful: Geranium renardii /cultivars