IO "Problem"?

I wouldn't worry too much about it, although that is rather high for a search thingy. Does it do that on each search, or just when it needs to update/rebuild its index? ("vmstat 1" will give you a quick one-line summary of everything each second, so you can have that up in a window while you prod things.)

Also, does this mean your vBulletin doesn't require MyISAM's blasted fulltext index? If so, I might have to look into that…

Yes, I believe that Sphinx does NOT use the FULLTEXT. The re-indexing takes place daily, but there's also some "delta" bit that gets run every ten minutes that has to do with it–that's where I assumed the IO was coming from, since it's steady throughout the day.

Sphinx is great for us--we've got about 5 million posts, and using the default Search with vBulletin gave us a lot of timeouts, rendered it almost useless. Most guides I read say that if you have such a large amount of posts, using Sphinx would probably be beneficial.

And thanks for the vmstat info--I will definitely use it while prodding.

Sphinx appears to be transparently scalable, and there seem to be vBulletin search modules that enable to to at least offload Sphinx onto a different machine than your application server pretty easily. So in the worst case, you can always spin off Sphinx onto a separate box.