I am currently an AmeriCorps member at the American Red Cross (although not a Vista member), and looking back on my half-year of service thus far, I can say that I am very glad that I chose to do this with my year before medical school. I target the underserved community by administering health and safety classes, such as HIV/AIDS, Community Disaster Education, Kid Safety classes, Babysitter's Training, Together We Prepare, and First Aid/CPR certification classes. My experiences have helped me become a more effective public speaker and leader and has opened my eyes to the huge need to improve service in the underserved area. In our program, the "underserved" includes all youth, schools, non-profit organizations, community-based organizations (e.g., churches), people without the financial means to pay for Red Cross classes, and those who do not have access to our services due to geographic constraints. My experiences have had such an impact on me that I am working towards serving the underserved when I become a physician.
Other things that I absolutely love about AmeriCorps is the multitudes of people that I've met. Not only do I work with other service-oriented/driven people (my wonderful AmeriCorps colleagues), but I simply love meeting so many different types of people in the community. I have taught classes at a non-profit organization where people were trying to get jobs, a homeless shelter, Boy and Girl Scout troops, inner-city schools, and even to the non-English-speaking Hispanic/Latino population (with my proficiency in Spanish, I actually had to translate my English scripts to Spanish and conduct the entire class in Spanish because outreach to the Hispanic/Latino community in my area is so low - talk about when my five years of Spanish classes finally became useful!). With AmeriCorps, you really don't know what to expect, but if you enjoy serving others and making a difference, I almost guarantee that you will have a learning experience that will have a huge impact on your life.
Each AmeriCorps member will have different experiences depending on the organization they work with. If you are more interested in people's experiences with Vista, perhaps you can go to the AmeriCorps website, search for the programs that interest you, and e-mail the contact person. I'm sure s/he would be more than happy to give you the contacts for their Vista members so you can communicate with them directly.
I know that in general, Vista members deal more with administrative work (e.g., planning service projects and writing grants), while regular AmeriCorps members do more "hands-on" work and actually go out in the community. I can't really say whether you should or should not sign up for AmeriCorps, though, because your decision depends on your interests and where your goals lie. Do you think you would rather do "hands-on" work or planning/administrative/behind-the-scenes work? How important is community service to you?
Hope this helps. Good luck!