I picked up a rather sizable Phyteuma vagneri at our Chapter plant sale (brought by Harold Peachey) and I without a clue about what to do with it. I've grown one or two small Phyteumas in the past, but this one is new to me. Any cultivation experience?
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.
Some grow very well in sparse grassy meadows and die down after flowering (viz: nigrum). You might just be able to make out the little clump of blackish flowers off centre in this picture, taken at Blackthorn Nursery. This is a grassy bank planted up with many species of bulbs and meadow perennials. Might take a while to make something like this!