[_pigpen__]

They’re sort of similar, but also opposite. A convertor twists up the pen pulling ink into the chamber. A piston twists down the pen creating a vacuum first. When it gets to the bottom the vacuum is exposed to the nib and the ink is rapidly sucked up. I definitely find the piston less messy.

[bayindirh]

Vacuum based filling systems are called “vacuum fillers”, like Pilot Custom 823.

A piston pen is exactly same as a converter one. For example Pelikan M605, Twsbi Eco, Lamy 2000, etc.

Vacuum pens are nice, but need more care during filling and need deeper bottles since the process is somewhat more violent.