A couple thoughts that might help.

1. The content you're transferring from your camcorder is digital, so the quality of the cable you're using won't make any difference whatsoever. The data is either there or it's not, quality won't change based on any part of the hardware capture chain.

2. What you're seeing on your monitor after capture should be pretty accurate to what's there in the .avi, so if it's inherently dark, something is most likely awry with the LCD monitor on your camcorder. I would echo Greg's suggestion, and perhaps offer another one: Take the SVHS or composite video out of your camcorder and hook it up to an external video monitor (CRT rather than LCD) and see what you're getting relative to what's being displayed on the LCD screen built into the camcorder. You should be able to adjust the relative brightness and contrast of the internal LCD to match what you're seeing on the video monitor, which may or may not be a close match to what your computer monitor shows.

3. If your specific problem is lack of crispness and detail on the final encoded video, that would definitely point to the codec and/or encoder settings you're using. It's always a tradeoff between file size, sharpness/detail, and how smooth the video is when you're encoding using any of the lossy codecs (WM, QT, Real, etc) and sometimes you have to tweak the settings some to come up with a good combination for the particular video you're encoding. For example, we shoot a lot of video outdoors, and that doesn't compress as well as video shot in, say, a hotel room because the leaves and other elements of the background are moving constantly and take more space to store than the white wall of a hotel room does.

I can't speak to the codecs built into Vegas, but I can say that Sorenson's codecs are, by reputation, of very high quality, and I've found that they generally do a better job both in terms of quality and file size (not to mention encoding speed) than the built-in codecs in Premiere. I've heard similar things about Canopus Procoder, but haven't used it.

Hope that helps!