I would bet that Waterman ink would be the standard for testing anything related to nib performance or barrel drying or staining of any kind.
Caran d'Ache Vibrant Green did that in my pen when it had been in the barrel a little too long, so I think it's related to drying of the ink in the barrel, more than anything. My Vac 700 was a pen that I didn't use often at first, but there were times when I did extended writing with the plunger opened to allow the ink to flow for the long session. Then I put it away again for a week or more before using it again. After a few rounds of that, I started getting the droplet problem, so I'm betting that it started drying up from that type of usage. When you open up the plunger, you're letting in air which will dry out the ink, like it or not.
That's why I don't use this pen as a standby writer. I use it often or I empty it and store it for later use, because it's not a pen that you can let keep inked for prolonged periods of intermittent use.
If it's not an everyday writer, don't overfill it and don't let the ink sit too long in it, even if you give it a small fill. Also, use a "wet" ink like Waterman or any of the Pilot lines, and avoid dry inks like Pelikan and Lamy. This isn't a bash against either ink, I happen to like some of their colors tremendously, but they are prone to dryness, and that won't serve this pen well.
Edited by Aquaria, 16 December 2016 - 23:40.