Actually V-sync will work at integers of the refresh rate depending on the performance of the available hardware. It could lock it at 30 it could lock it at 20, most likely it will jump between different integers, either way its worth a go.
Don't know where you got the idea it will only work at the refresh rate of the monitor but wherever you got it from its wrong.
You could say it will only work properly if your hardware can exceed the refresh rate of the Monitor, that would be correct if that's what you meant.
Its still worth at least trying.