The error code regardng an O2 not fluctuating is mose likely the rear o2. if you scan the ECU with an actual scanner it will detail which o2 is throwing a code. the Rear o2 will affect your ignition timing slightly but not enough to throw your fuel mileage off.

Think about this logically. if the vacuum is low like when it is supposed to be 18-22, not 17, then how is the ECU going to react. The ECU will think the engine is getting more air and inject more fuel. This would lead to bad fuel mileage.

Now what could cause low vacuum. Vacuum line leak, i doubt this since you replaced all of them. Gasket leak since you just replaced the head; spray carb cleaner around all intake gaskets while the motor is running. Any change in RPM will indicate a leak. Another distict possability is your cam timing. if your cams have 1 tooth of overlap then you will deffinitly have a loss of vacuum. Aftermarket cams will also create this condition. Anyone who still wants to claim cams wont cause this need to find a new hobby.

So without knowing what exactly you have in your head that was replaced recently i would say check for leaks then chekc your cam timing. If you have aftermarket cams you will need adjustable cam gears to decrease the overlap to raise your vacuum.

Terry