Okay, I'll try to be brief and concise in describing the setup and problem encountered - hopefully someone will have answers or leads before too long, as this problem is really costing a lot of time.

I have a new 64-bit Vista OS running both Vegas 8.0c (x86) and 8.1 (x64). I have installed both the 32-bit and 64-bit versions of HuffYUV 2.1.1 and installed the 32-bit version correctly as described here:
http://www.dvinfo.net/conf/what-happens-vegas/126319-installing-huffyuv-vista-64-bit.html

Using Sony Vegas Pro 8.1 (to fully utilize my 64-bit OS and quad-core processor!) I was rendering to HuffYUV just fine - until just a few days ago. Now HuffYUV won't let me configure - I hit the "configure" button when setting up render specifications, but nothing happens; no usual pop-up config window.

I suspected Shark007's x64 codec pack, which I'd heard was "safe to use", was messing around with HuffYUV - but, having removed this and uninstalling, then re-installing both HuffYUV64 and Vegas 8.1, I find this problem still getting in the way.

I'd be very happy to get any help whatsoever on this!

I'd try doing a system restore back to when it did work.

A friend actually just sent me a message saying he had the exact same problem and that Sony was to blame. Rendering to AVI with HuffYUV works, but the configurations cannot be altered. I assume it renders using the predict median (best) setting, although I haven't experimented yet.

I may well have had deluded myself that it was working earlier - I must have been thinking of 8.0c.

I never did get huffyuv to work on 8.1. Changed to Lagarith which worked in 32bit and 64bit, but abandoned both to use Avid's DNxHD. Takes less space, clean, works cleanly in 64 and 32 bit, works on Mac and PC.