The hole for the vent doesn't exist on the barrel when you get it. It must be located and drilled by the builder.
A word of caution here:
The location for the vent depends on the location of the breech plug face. It must be forward of the breech plug.
With this in mind, the lock mortice will already be roughed in so the locks location is fixed.
That leaves the barrel as the only thing that can be moved to position the face of the breech plug at the proper place, relative to the center of the flintlocks pan.
The recommended proceedure for building one of these pre-carved stocks is to first, finish inletting the lock. (They do not fit when you get the kit).
Once the lock is inlet, install the breech plug. (I recommend paying the small fee to have the supplier do it).
With the breech plug in place measure the exact distance from the muzzle to the face of the breech plug and transfer that location to the outside of the barrel where you can see it.
Measure the diameter of the vent liners threads and divide the size by 2. Add 1/32" to the value and mark a line on the barrel that represents this dimension.
That line should line up with the center of the pan on the lock.
Usually, the stockmaker will leave extra wood at the end of the barrel channel so your job will be to remove this wood, moving the barrel back towards the butt until the line on the barrel lines up with the center of the pan.
Sounds like fun doesn't it?