Vivianite crystals build up only if phosphorous acidic solvents are in contact with Fe-compunds like Pyrrhotine, Pyrite, Siderite, Fayalite - in an oxygen-free environment.
First location had been Nickenicher Sattel, later also in Dellen area in contact with Fe-rich environment with Jarosite. (quoted G.Blaß)
The micromounts are very fragile and have to be handled very carefully. The following pictures are linked to findings from Bernd Ternes, thanks to him.