There are indeed two versions of the binding:
Assuming you’re running the latest 2.5.x version of openHAB, you want to install the Velux binding with the highest version number available to you!
Checking the documentation, the Velux binding is flagged as V1, but shows configurations for openHAB Things - the concept of Things didn’t exist in V1! Not sure why it’s confusing the two versions.
Here is the (long) forum topic for the V2 binding: