My truck has another problem this week.  HVAC is stuck on defrost, the vacuum pump produces vacuum, and all the hoses are connected.  How much vacuum should I be seeing at the vacuum distribution plugs near the heater output?

there is some oil leakage through the vacuum pump, so is this causing poor vacuum?

Are you positive the small vacuum lines in the engine compartment along the firewall are all connected? Small hard plastic vac line comes up from vac pump to firewall on driverside of engine and tees into a manifold of sorts, one line goes down back side of engine and along top of trans to supply vac for CAD system on 4wheel drive and the other lines go through the firewall and are what supplies the selector vents with vacuum

Double check all your vacuum lines I've seen cases where they have melted closed or dry rotted and crack open but the crack is so tiny you can't see it. ISX posted a while about how the little rubber boots would leak too.

I changed the little rubber boots and connectors on mine to clear gas line that we use on snowmobiles. It doesn't suck down and should last just as long as the rubber.

I used cheap clear vinyl tubing on the 1996 Dodge for replacing the vacuum tubing. It works but if routed near hot parts it does suck flat. So the fuel line would bee a bit better.