If you have XCache or Memcache installed on your server, you can choose either one in the vB Optimise settings, and vBO will use that instead of File Cache, as both are much better than using the file system to cache data.

Do you still have to enable xcache in your config.php if you are enabling thru the mod?

XCache does not need to enabled through config.php, nor does Memcache, but the memcache server info does need to be included in config.php :)

So if I have it enabled thru config.php, should I comment out?

Not at all, you can use XCache in config and for vB Optimise, or use Memcache in config and XCache in vB Optimise, it won't conflict :)

Thank you kind sir :)

No problem :)

How well will this work with SEO or crawlers?

Won't effect either, this is purely improving performance. If anything I guess it means spiders won't add as much load as they used to :)

I get this when trying to flush the cache:

Fatal error: xcache_count() [<a href='function.xcache-count'>function.xcache-count</a>]: xcache.admin.user and xcache.admin.pass is required in /home/xxx/public_html/forums/includes/class_activecache_methods.php on line 161