No, mostly you can't go wrong with HMS - provided that Steps, LoR's etc. etc. are also competitive. Vanderbilt certainly also has a good reputation. Happen to know several happy people who did derm rotations there.
However, the real question in your case, I think, is one of medical school admission. I assume that you have a non-US undergrad. degree, and that DOES make it harder to enter med school. In fact, it's probably close to impossible, unless you have SOME US college experience.
The facts (since you mentioned Harvard):
"Foreign students who have studied for at least one year at an accredited institution in the United States or Canada, and have completed the requirements, are eligible to apply at HMS. International applicants should make every effort to take the majority of required courses at an accredited college or university in the United States or Canada.
Foreign students who do not have a baccalaureate or advanced degree from an institution in the U.S. or Canada are rarely accepted for admission."
You can also take a look at AAMC statistics (
http://www.aamc.org/data/facts/2004/2004slrmat.htm) but they don't really give you any hard numbers.
As regards visa issues, those are also getting ever more complex. Yes, you can study on an F-1, but you can't work on one. For that you need an J-1 (most common) or employer sponsored H-1B. And if you're a J-1 resident, you WILL be required to go home for min. two years after you finish your residency.
Finally, if you DO get into med school, remember a water-cooled credit card. There's NO federal loans, and often severely limited financial aid for non-citizens. Even postdoc, NIH grants are reserved for citizens or green card holders.
Don't want to say it can't be done. But all in all it's actually probably easier to take the IMG route, i.e. take a non-US degree and go to the US. That, however, all but rules out a future in dermatology.
Other option is to take a graduate US degree, and THEN apply to US medical school - which is probably most viable option.