I have had two clients with vertigo. I agree with Pandoras gift that a subluxation can cause it. I also agree with Maestra that an inner ear imbalance can cause it & PT can be effective (if it's someone who specializes in balance problems). As far as massage goes, definitely focus on the SCM, but also the suboccipitals & posterior neck muscles (especially along the upper cervicals). I consulted with a PT and she said balance problems stem from the upper neck more often than from tension in the SCM. (Which fits with what Pandoras_Gift has experienced chiropractically).
Both of these clients were regulars who I saw weekly, and although I did detailed medical massage on the posterior & anterior neck muscles, the vertigo didn't disappear until I learned Lymph Drainage Therapy and added that to their sessions. I don't know if you're trained in that or not. If so, definitely use it. Otherwise, I'd recommend either chiropractic or PT consult in addition to your massage sessions.
Good luck!