Have to disagree. Blizzard has a subscription service for WoW:WOD. It offers no such service for WoW:Vanilla. You could argue that they are the same game but experientially they are completely different.

Except the current version of WoW is only what Blizzard supports. It’s impossible to have a Vanilla experience in the officially supported game. It’s entirely reasonable to say that Vanilla WoW is not supported at all by Blizzard and it is impossible to play it except through private servers.

They would never pay the subscription in the first place. They deliberately joined a server running the oldest version of WoW available. There’s no reason to think that they would be playing the retail version if they couldn’t play Vanilla. They want to play Vanilla and only Vanilla.