Sounds like a white lightning liner, we file them flush after they are seated in the barrel, they have no slot. To remove on you have to drill a bigger hole in it and remove it with an easy out. You would only remove this type of liner if you plan to put a new one in, removing it destroys it.
Another question. Why do you want to remove your vent liner? You should only need to remove a vent liner if there is a problem. Cleaning is not a problem that I consider to justify removing a vent liner.
The only reason mechanically to remove a liner , is if the touch hole gets oversize and the accuracy begins to drop off. In years back , some guys began to use a 10 X 32 removable Allen lock screw with a hole drilled in the bottom. The idea was to make it easier to clean the rifle. Well , the constant heavy handed wrenching on the A-screw eventually ruined the threads in the barrel. That was enough reason to quit using A-screws for touch hole liners. Most m/l barrels are lead bearing steel alloy and can be soft ...............oldwood
There's really no need to remove a liner unless work is being done and removal is necessary to proceed. I've rarely removed mine (slotted) and only for work and never for routine cleaning, etc. I bought several extra liners years ago and still have every one of them in the original package.
The White Lightning liners are made to give you more powder right at the touchhole for faster ignition.
They are made like this, we cut off the lug, it is only for grabbing with pliers to tighten the liner in the threads. You can see the counter sunk shoulder that fits just inside the barrel for a good seal. Once seated the liner face is filed flat with the barrel.