I'm not even convinced the odd sorting is that much of a bad thing. It's actually rather logical and more relevant, if you think about it. For example, you have 3 threads and you're searching for the word "cheese":
Thread 1, a thread posted to on a daily basis - the word "cheese" was posted 1 year ago.
Thread 2, a thread posted to on a weekly basis - the word "cheese" was posted 1 month ago.
Thread 3, a thread posted to on a monthly basis - the word "cheese" was posted 1 week ago.
vBulletin search would likely show the threads in the order 1, 2, 3.
Sphinx would likely show the threads in the order 3, 2, 1.
On the face of it, thread 1 hasn't been about "cheese" for a year, despite it being regularly posted to. Wouldn't a searcher looking for "cheese" find either thread 2 or 3 more relevant?
I would suggest the sorting issue with Sphinx is more about people's expectations than any actual problems. The results of a search, even for more common words, might actually be more relevant in most cases.
Just something to think about.