Reaper seems to be working great for me (so far), but I switched over to Windows to use it. From what I understand it's not as mature on OS X. And this is coming from a DP guy, though I might switch back once it's out for Windows.
I'm actually a die hard Mac guy, but I can't deny that the latency is much better in Windows (with the exception of maybe Logic). I have to say though, a lot of people don't know about or give DP a chance, and it's probably one of the most solid and intuitive DAWs out there, and works great with VE Pro. I guess I've just been getting a little DAW ADD lately.
I've found Reaper to be completely unintuive, and honestly tried it 7 times before I finally dug into it, and now there are quite a few things I really like about the way it works. It's the little things. But, I digress.
So far I have 3 instances of VE Pro running from a Mac slave, and it's been fine, but I haven't really tried to write something huge just yet. I will keep you posted.
I did have a latency issue very similar to this once, but once I shut down VE Pro and reloaded it, it went away.
I will say this. I've never been much of a fanboy, but I am a VSL fanboy through and through. They could have very well just stuck with samples and not done anything else, and they are really pioneering the way. VE Pro 5 is hands down the best musical investment I've ever made, and I love it every day that I use it.