Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3442983.

The Vertex Delta from Velleman comes with very unusual and deceitfull belt pulleys. Basically 3 bearings on an M5 screw. The belt tension applies a force on the M5 forcing the belt to slowly move back and forth on the bearings and inevitably, even with the best aligning and tensioning, you will end up with the belt jumping out as nothing ensures it to stay.

The stock pulleys are very hard to tighten and it's hard to apply tension to the belts because of the way it's made.

Here comes the pulley upgrade!

Simply print three of those, unmount the stock pulleys, keep everything and use it to make this pulleys. You'll need to buy 9x M8 bolts and some M8 threaded shaft