Marco thanks for taking the time to go through this I really apprecaite the guidance.
Your first example is exactly how I want to setup my forums. I have a forum that is related to two mobile phones made by the same company. I have a main forum with two categories for each different model of the phone. I have used your example below and edited it to show my intended setup.
- General Chat -
www.mainmobilesite.com
- Topic 1 category
www.mobilebrand1.com
--- Topic 1 subforum 1
--- Topic 1 subforum 2
- Topic 2 category
www.mobilebrand2.com
--- Topic 2 subforum 1
--- Topic 2 subforum 2
As you have advised all the navigation in the header and footer leads back to the main site "www.mainmobilesite".
If that is allowed I am very happy

, I took the dive and bought the forum software anyhow as I was going to do a 301 redirect from the domains I had but that was not the preferred option. If I can set it up this way without breaching anything then that is great.
Now for the million dollars questions....
1) If I point a category to a domain how do I configure that domain on my host and vb setup? I know how to use URL's to make a category but how to make the domain display the category if that makes sense? Say I have
www.mobile1.com
How do I make mobile 1 display the category it needs to? Do I need to park all my domains onto the folder where the VB install is? I have a VPS so I have root access and know my way around a shell?
2) I am assuming this will not harm SEO as there will not be any duplicate information?
3) How does login / logout work passing between domains? I saw on the parrot site that once you visited a category a java script would pop up asking you to login again and it would automatically login you into the new domain? Is there a hack to do this? I saw cookie cutter for 3.7 but I am on 3.8.
4) How do I ensure the new posts section is updating across the domains? I am assumning that would come from answer 1??
Thanks again for your help I am really glad I took the dive in, I have setup a test board already and imported all my users successfully and its a really nice easy board to use.