9 out of 10 use a 'tubing handle' for the vang in the middle. The two lines left and right are usually not 'tubed'. That has to do with the way people rig their Laser: the vang is a complete set and the line doesn't have to come off every time. The left and right control lines are usually tied (or spliced) to the blocks at one end and the other, free end of the line has to come through the blocks on the deck when you rig the boat. That's why most don't use a tubed handle, only a braided handle.
Tubing: choose a piece for Harbor Fright or similar. The cheapest one will do. Length? According to what you prefer!
One suggestion: when changing the control lines (including the vang), try to 'upgrade' to different colours with a lot of contrast between them. My son currently runs (from left to right when sitting in the boat): Orange - Black with tubing (vang) - Lime. I'm sure that you'll see the advantages when grabbing for a line in the heat of the battle near the buoy!
For an elegant set-up, proper advice from a seasoned sailor and A+ material, turn to Max Hunt in the UK
Well... I'm sitting 'on the dock of the bay' so to speak... Foul weather outside. Despite that, the national youth squad is out on the water and I wait for their return. I'm inside a harbour restaurant with my MacBook in front of me. That allows me to respond quickly. I am the designated driver! (17 y/o kids are allowed to drive here - provided they have a license but need to be accompanied by an adult up until they're 18 y/o - in our case: August)