Club Lexus isn't the only game in town. The Lexus Owners Club is also very well established with three forums (a very large USA forum, a fairly large UK forum and a new Australia forum).
What would I do? I'd setup the lexusnation.com domain to point to your Lexus forums right here on Toyota Nation. However, I'd update the templates to give the appearance that the Lexus Nation forum stands on it's own. I did something similar, though I rushed it and didn't do a very good job
here. If you go to the forum on 90sxchange.com, it's actually 80sxchange.com's vBulletin.
You can easily create a custom header and update all the templates to make it look like the forum's front page is independent.. Create separate sections for individual models and go from there. Am I explaining myself well enough here? :p
The reason why I'm suggesting this is because you already have a very good thing here that's all-Toyota encompassing (Toyota, Lexus and Scion). Why break it up if you don't necessarily have to?