I am a current Teach for America corps member and I considered vista before accepting my offer from TFA. I can tell you that you will not regret doing a year or two of work with AmeriCorps. It is an incredible experience and even when you are done with your education and begin working on the policy side of your area of interest, you will probably never feel as involved in the process as when you are working in VISTA. Not to mention that your GRE/GMAT scores will still be good by the time youre done. VISTA will only help with your eventual admittance into grad school.
Like you said, you will have an additional year to take the GRE. Your application will be more competitive. You will be given an AmeriCorps education award.
I say go for it!
To answer your last question, I really don't consider 30k to be TOO much entirely, especially if that masters will check all the boxes you listed: top choice, regional preference, girlfriend/personal life wouldn't be negatively impacted, existing network etc. And you may be able to work while you complete it to boot. I will probably finish my masters with a little over 30k total, and I am not overly concerned.