But I wanted to have a vacuum filler for long, uninterrupted writing sessions.
Experience with many different filling systems has led me to conclude that the quest for ever larger ink capacity is folly.
Your common converter holds about 0.6 ml. Many popular piston-fillers have capacity about 1.0 or 1.1 ml. To me this is a healthy quantity of ink.
The Vac 700 draws about 1.2 ml with a single plunger stroke, so that's not any great improvement. If you use the special filler bottle (which is actually pretty nifty) to fill the whole barrel, then you're looking at about 2.0 ml of ink. However, I never was able to solve the flow problems with my 700s, which meant that I still had to jiggle the cap once in a while. So that's not exactly "un-interrupted" writing.
The Gate City Belmont is a syringe-filler that draws 1.7 ml with a single plunger stroke. It's easily disassembled for cleaning, the entire ink supply is visible, it doesn't require any jiggling or fiddling to keep the ink flowing, and it doesn't break. To me, it really lived up to everything the Vac 700 seemed to promise. Unfortunately, the Belmont was also rather more expensive, and was produced in modest numbers, and is now out of production. So it goes.
My Edison bulb-fillers hold about 2.5 ml of ink. However. . . They have proven to be somewhat finicky, and there are some inks I'm afraid to use in them because of the rubber bulb.