I just installed a boost gauge in a 02' vw Jetta 1.8t, at idle or low rpm (below 2k) the gauge marks 20(IN-Hg) vacuum but once the car starts making boost the gauge reads up to 15psi of boost.

I'm assuming there is a vacuum leak somewhere, but I just want to ask those more familiar to boosted cars. is that normal, or bad?

info:
1.8t jetta
stock turbo and innercooler
cone intake filter

Its normal

thanks, this is the first turbo car I do work on.

any car should have a vacuum reading idle/low rpm or it just doesnt idle. unless you have aggressive cams or a bridge/peri port. id explain it further but im pretty ****ed up at the moment LOL

+1 on normal. If you were to put a boost gauge on an N/A car, it would behave pretty much the same, vacuum at idle and it would approach ambient pressure as you opened the throttle more and more.

Yep, normal. And on decel, especially after you've just gotten on it, it will go way into the vacuum side, hehe. When just cruising you'll be a little into vacuum or at 0 depending on if you're going up a hill or something.

If your gauge is reading about 20inHg at this elevation, that's pretty strong IMO, assuming your gauge is accurate. Generally, most cars at sea level run 18-22inHg, but at this elevation, most people on turbo Subaru's seem to see 15-18inHg (~7-9psi) at this elevation.

Generally, the amount of vacuum you're pulling at idle is indicative of the compression of your motor to an extent, as well as vacuum leaks (among other issues) and with 20inHg at this altitude, I would say that's a healthy motor. The only thing that would worry me would be if there was a significant fluctuation in your vacuum at idle. Here's a pretty good guide about trouble sign indicators with vacuum at idle:
http://www.imperialclub.com/Repair/Fuel/VacuumGauge/index.htm

yeah i see about 15-16 on average at idle and a wee bit more than 20 when decelerating and engine braking.