Rotary vane vacuum pumps use a sliding vane to create a vacuum within the pump and tank. If you have a Wallenstein, Masport or the like, you will have vacuum pump ‘vanes’ as opposed to the human body’s ‘veins’.
The vacuum pump vanes are offset within the housing and slide in and out of the rotor as it rotates in the housing. The vanes make contact with the housing throughout the entire rotation. As the vanes rotate, they wear ever so slightly. All vanes have a tolerance which once reached will need replacing. In the case of say a Wallenstein HVOA 753, the vane starts life in sunny Canada at 4.5" wide and when it gets to 3.5" wide it is time to replace it. The actual vane tolerance is 1" but personally I would replace it sooner rather than later as it is much cheaper to replace a vane than potentially a complete pump.
The vanes are usually made from carbon fibre, however, in the olden days they were made from an asbestos compound so it is very important to be careful if handling an older pump which hasn't been worked on for some time.
When a vane has worn past its tolerance it is able to fall off of the rotor slot. This is catastrophic as the other vanes will catch and then the most likely outcome is that the rotor will shear a lob before breaking the housing and end plates - a little like a train crash. It is simply one after the other after the other.... Painful and expensive.
Checking vanes is generally not a long process. In a Fruitland pump it is possible to check the vanes without pulling the pump apart at all. In a Wallenstein or Masport, the end plate needs to be removed which can take up to 30 minutes depending on the pump and its location. Once the vanes have been checked and measured, the inside of the pump should be sprayed with light machine oil to ensure the vanes slide freely in and out of the slots before reassembling the pump. Reassemble as per the Owner’s Manual and you are right to go.