Not all programs use VSAS... So if you're particularly interested in a certain program, check out their residency site for deadlines (google works best).
As for timing, well, interviews typically start in November, and you want at least 2 SLORs & plenty of time for them to get done... So, July-August-September, IMHO, would be the best bets. October cuts it close... But is an option. Try to avoid them later than that.
Another option is June if you can; seniors are unloading procedures 2/2 senioritis, and the interns are much better seasoned, so minimal competition for attending attention... Caution, though, you need to be strong as this is a double-edged sword. More attention = higher likelihood for recall if performance is weak.
Good luck!
-d