Removing that liner is easy, and you have received good advice from posters.
Drill an eighth inch hole in the old vent. Use a small easy out, turning it courterclockwise, and the liner will either back out, or strip the metal of the easy out, depending on its quality. If it does, use a bigger easy out...they come in sets. The old liner is an Ampco beryllium copper liner and yes, it looks like brass. It is as hard and tough as mild steel. If you ruin all your easy outs, drill out the liner with a number 3 drill bit, chase the threads with a 1/4" x 28 tpi tap and install a new Ampco vent. But better yet, buy Chambers 5/16" x 32 tpi liner and the appropriate drill bit, and install the new improved liner following Jim's instructions.