In HDLink in preferences under CineForm Encoder Options there is a check box for I frames only. I would like to know what that is for. I didn't see it in the manual. Thanks!

It was called "smart rendering", but that is more obfuscated than I-frame mode. All it means is frame as store separately within the AVI/MOV. It is useful for 3D projects, and lossless smart renders in Vegas. Leave it on if you don't understand the need, turn it off you want to save 15% disk space for the same quality.

So leaving it off has the same quality as having it on? I use vegas, whats the benefit?

Same quality yes. In Vegas you go lose smart rendering.

I use Vegas and I love the smart-rendering, but now I'm wondering:

If I use First Light for my color correction, I'm losing the ability to smart-render anything that I've adjusted the color on. Is that correct? I might as well uncheck it and save the space.

Here's another question. When I do documentary stuff, I light the interviews, set white balance, make sure everything is perfect before I shoot it. I rarely need any sort of correction.

However when I shoot B-Roll, I use a lot of auto settings and a little color correction helps quite a bit.

Any problem mixing I-frame and non I-frame footage? Does Vegas just smart-render the I-frame parts and fully render all the non I-frame parts?

Yes, No, and Yes to your questions. All will work as planned.

There's no "I-frame" setting in the vfw codec settings that Vegas uses. Am I correct in assuming that anything rendered in a vfw application like Vegas or VirtualDub is automatically using I frames?

Yes, this has always been the case. Creating long-GOP (even two frame GOP like CineForm can be) never worked with VfW tools. VfW and QuickTime are treat as I-frame always, whereas DirectShow didn't care.