Great question. I actually cover this topic in a 2 hour CE presentation, and it is always generates alot of questions and comments.
Ok, although there are some exam material differences, as well as some pre-req considerations, the truth is there isn't a nickles worth of difference between any of them. They all effectivly test you over the basic content knowledge required to be registered vascular sonographer.
Although many may disagree, the reality is that one is not really better than the other in way. I once held the RVS and now hold the RVT and the test were very similar in content. Additionally all are recognized by all societies ultrasound as well as all vascular lab accreditors.
That being said, the ONLY difference is in the eye of the beholder. What I mean is that people, manangers, and facilities will prefer one over the other based on their own subjective reasons. And subsequently the RVT still remains the most "accepted or prefered" credential of the lot. The ARDMS has done an excellent job of branding themselves as the ultrasound credential of choice. It is starting to shift though.
BTW, if you are RVT....then congrats. You hold one of the most coveted imaging credentials. I see no reason to add any additional vascular creds, unless you just want to for personal reasons.