Since there has been no answer here, I thought I'd mention that Graham Nicholls says, in his Dwarf Campanulas that it is very striking, due to its blackish violet flowers, but warns that it can self-seed vigorously. 
I haven't grown P. vagneri but I find that the ones I do grow (P. scheuchzeri, P. charmelii, P. nigrum... I'll have to check if I still have the latter - the best of the bunch - or whether it has been crowded out) seem undemanding.  I have them in regular soil in sun to part sun.