Correct vacuum reading for 1995 C36?

[MaxC107]

I purchased a 1995 C36 with a blown head gasket. I fixed that and the car runs fine at speed. However the idle is low and rough. I checked manifold vacuum by hooking a gauge up to the port on the top of the manifold where the 180 degree vacuum line with about a 8mm ID normally goes. The gauge read 15 inHg which sounds low. Also worth noting is that this car sat for 2 years before I got it last week. I have changed all the fluids including fuel but have not done plugs or 02 sensors since they looked fine. Compression is good on all cylinders after the head gasket.

I was able to drive the car into my garage before the head gasket job and it did not idle as rough.