Excessive vacuum in crankcase

I have truck listed above with a 350 and there is excessive crankcase vacuum and the Idle is high at 2,000 rpm, there was only one code for a misfire on cylinder four.
When I checked that one there was a good amount of deposits on it. I did a compression check with all being around 165 lbs except for that one which was slow to come up to 165 lbs. If you take off the oil fill cap when it is running (if it does not pull it out of your hand) it just about goes to a stall.
Any help would be appreciated.

Thanks