Vacuum controlled systems (such as climate, or cruise)

I've been tying up some loose ends on my 90 and lately its been raining out, but my defogger isn't working. I realized that I haven't plumbed the vacuum line for the climate control, but as I was hooking it up, I started wondering what effect boost would have on this system and its function; Is this an issue? This used to be all vacuum, but now its going to be subject to positive pressures on the order of 20 - 30 psi.
Any thoughts?
Thanks!