ChrisWilson - 2:51 pm on Oct 3, 2013 (gmt 0)
I run two, very large forums with this exact model and use the built in features in vBulletin to accomplish it. You will not run into any issues at all so long as you keep the content exactly the same. In other words, what Google see's should be exactly the same as what the users see's even if they were logged in.
I actually limit the views to 5 threads, so the user coming from Google can actually see five threads, all exactly the same as Google see's, until they are asked to register.