On vanilla rooms, you can only play the official maps. On the March 26th update, user-created fixed maps were removed from vanilla rooms.
Vanilla rooms are defined the same way as bootcamp rooms. As long as "vanilla' is in the title of the room it will behave as a vanilla room. It is possible to put any character in the /room name, as long as vanilla is part of the text string, for it to be a vanilla room. Pressing the Vanilla option in the menu takes you to room vanilla1 or vanilla2, in a similar fashion like logging in. If too many players are in vanilla1 or 2, new rooms with name vanilla3 and following names (vanillaX) will be created and mice will be put into these.