I suggest that you take the Vanderbilt offer. Upon reviewing your math background, you have enough math courses to give you a legitimate shot as a PhD applicant, so any additional math courses that you might take as a math major would yield relatively low marginal value in your application. Instead, your eventual PhD application can benefit a lot more with some RA experience, which you mentioned in your post that you are likely to get (or at least more likely to get than extending your undergrad by a year) if I were to take what you said at face value. And I have no reason not to. So I think this is an easy call based on the information you have provided. Of course, if you were to tell me that instead of attending a typical undergrad (which I assume you are), you are actually at one of the top 10 US universities for undergrad education (based on say US News Ranking if you will), then yeah, get that math major.