I realise now that there are two sections to the adventure box at the top of the menu (left and right), and the reason I thought that the left part of the box lead to anything other than the main vanilla story, was because the introduction didn't resemble that of the demo. I can confirm that I was wrong and I am now able to play the vanilla story line.
The demo is slightly different to the vanilla story.
Play in a little bit and you'll see that it's the same thing, they just cut out a couple of things in the begining.
To access Vanilla: choose Adventure (left side)
To access The Stars are Left: choose the right side of adventure (The Stars are Left)
To access Vietnam: look in challenges for Vietnam mission (for the adventure-like mission) or the Vietnam survival map (for the challenge).
All other DLC is either a challenge map or a pvp map or simply robes or items.