Yeah, I have a VDO that goes from 30in Hg (vac) to 25 psi (boost) I have only ever seen it go to just less than 5in Hg vac when I was backing off from 135km/h. So yes the vac part is probably wasted. Normal driving it is between 0 and 15 psi most of the time. I can still read it OK when its at 15 to 18 psi but it would be nice to have a bigger scale (or bigger increment spacing I mean).
If you can get one at a reasonable price with 0 to 25 or 30 psi then go for it.