From my experience,not very many things stay the height stated on the tags in plant stores. i 've never heard of a compact vitex. So I bet you will be fine.

I agree. Most shrubs get far larger than the height stated on their tag. If it says 12 ft it will probably get taller. However, I looked on Google and there is a compact cultivar named "Bo Diddley" that they say gets only to 3 to 6 ft. and another one called Pink pinacle that grows to 3-4 ft. Those might stay smaller. Otherwise, since vitex tends to get fairly tall and sprawley the tag is likely to be hopeful hype. Just a guess. Probably nobody knows for sure, but even a slight bit more of a compact selection might be a good thing.