First thing is, you should identify the cause. Is what I mentioned really the case? Did you really record with full color range? And if you feed Sony Vegas with partial color range, are the colors really correct? You know if the former was the case, and the latter can be verified if you create new footage with partial color range in the first place and check that footage with Sony Vegas.
If you verified this, and this is all the case, you need to either make Sony Vegas interpret the old footage as full color range, or convert your footage from full to partial with some 3rd party tool. I cannot help with Sony Vegas, I don't know that app.