Having participated in the match for internships (and starting the internship in about 2 weeks) this past year, I think I have a fair idea about how unmatched spots work. The practice I matched at only matched 2/6 spots. The practice is given a list of the people that did not match (in the appropriate field- small animal practices obviously won't be interested in people who applied for equine internships, etc.) and they contact whoever they want from the list. I have a classmate who didn't match. Within 2 days of the match date, she had over a dozen e-mails from practices that were interested in her. She did research on those practices and selected a couple she liked. Then she had to e-mail a copy of her application to them. A few days later, she was set. Just keep in mind, you can only be considered if you are part of the matching program. You can't be an outsider who heard a place has open positions.
In terms of derm, there were 7 universities offering positions last year (with 8 spots available). According to VIRMP, all but 1 matched candidates. As it turns out, 4 of the universities have 2 year programs (FL, CA, MI, PA) while 3 have 3 year programs (IL, NC, TE). I looked at the websites of a few of them and I have no idea why they aren't consistent with program length. I guess you'd have to look specifically at each program and figure out the pros and cons before applying.
I hope this helps a bit. Let me know if you have any more questions about the match process or internships, etc.
🙂 Judy