The expansion pack and multiplayer

Sorry if this has already been covered elsewhere (feel free to link me to the thread if so), but I was wondering how multiplayer would work between people with the Xpac and people without. From what I understand, the map is the same as vanilla except with an Act 5. Will you be required to select it as a different modes in the main menu? Can a player who doesn’t have the Xpac join a multiplayer match of someone who does? Will it be a separate tab in the server browser? How much will vanilla players feel like the plebs they are when my Inquisitor joins their session?

if you have xpac you can not play with vanilla people. GDX will function the same way as Crucible DLC or any mods basically.