Nikki,
I don't have experience with the 2 vessels, but I do have experience with testing. My baby's nuchal scan came back in the high risk category. Because of the increased risk of miscarriage and the fact that we wouldn't terminate, I opted to not have an amnio or cvs. We waited until our 17 week "big" ultrasound - this is when they can scrutinize all of the baby, organs, etc for any abnormalities, or markers for certain syndromes. Fortunately for us, our baby was deemed "normal", although they do stress that it is not a diagnostic conclusion, like it would be with an amnio or cvs. However, it was adequate for our peace of mind. Best of luck to you! :)