On older 850s, even though the code indicates a vacuum leak (EGR flow) it's usually caused by crusty oil deposits clogging the EGR port in the intake manifold. That assumes that there are no vacuum leaks to the EGR controller and/or valve including the vacuum T under the throttle base cover.
To clean out those oil deposits, you need to remove the throttle base and EGR valve. Once there, you'll see the hole the the intake manifold that's probably clogged solid. You then need to use tools (drillbit, etc) to clean out that hole. It's also a good idea to be sure the flame trap and its related vacuum lines are clean as that's usually the cause of excess oil being there.
The other problems are probably not related to the EGR but some moisture in the crankcase and poor mileage could both possibly indicate t-stat problems (engine running cooler than normal).