I would start looking at programs you want to rotate at in January of 3rd year. Reach out to them to express your interest. Before applying on VSLO, I'd recommend contacting the program coordinator or visiting student person and asking if they will (1) actually be offering the rotation, (2) actually offer it to outside students. A surprising number of programs will allow you to apply on VSLO and pay the AAMC $15, only to find out that rotation is not even offered or it was never even open to students from outside schools. Many programs won't give you any information or tell you the bare minimum: "Thank you for your email. Our application opens on this date." But others will actually hook you up and work with you to get you a rotation. So, for me at least, it was worth the time to send a few short but nice emails to try to make contact with a real person.
Most programs won't open for application submission until March/April or even into May. Try to figure out what you will need in terms of immunization records, background checks, etc and get that all set up so that you can submit as early as possible. There are programs that don't use VSLO, they can be high yield especially if you inquire early. Good luck, let me know if you have more questions.