One other point that may have been a simple omission, but I thought I'd mention it.

You refer to putting a sRGB -> cRGB filter on, but don't mention taking it off before rendering.

The DV codec expects sRGB input, so if you've left your filter on, you're crushing your blacks and whites.

You should ONLY have it on to perform the colour correcting/grading and then remove it before re-encoding. Forgetting to do so will result in output that is not what you expected, albeit perhaps subtle.

This is why it is preferred to not have to use the filter, and use an external preview device, or a secondary monitor full screen displaying in sRGB colour space instead of cRGB.

The "bible" of Vegas and colour spaces/levels, etc:

Color spaces and levels in Sony Vegas 9

Hope this helps,