They fit the original vulcan i.e the I.
If the rucksack you're on about is a Berghaus Cyclops II Vulcan then yes in all likely hood the issue PLCE side pouches will fit. As will any side pouch with PLCE configured Zips..
They did not fit! The zips were up-side down and when I tried to zip the pouch on up-side down, it id not work so I guess they are different zips even though they look the same.