For problem (1) Check your vaccuum lines for leaks and look at the air
intake breather hose for cracks.  Some in the GTi16V world say that while
the engine is at idle you can spray carb cleaner around the engine bay near
possible vaccuum leaks and listen for increased idle to pinpoint leaks.  You
may not like spraying carb cleaner around a running engine so just replace
all the critical vaccuum lines around the motor for cheap and eliminate
them.  Also loose or worn motor mounts will cause the breather boot to flex
a lot and possibly crack it causing frustrating idle problems.  You have to
take the breather boot out and really look at it good to find the cracks.
New ones are expensive probably but Shoe Goo can fix it just as well.  There
is also the injector O-ring source of vaccuum leaks.  If you haven't changed
injector o-rings in many years, they will become brittle and crack causing
more idle problems.  Use the Viton green ones.

Another cause of this problem, especially when you mention cold and wet, is
ignition stuff like dizzy cap cracks (lets in moisture).  Spark plug wires
burnt or dying can cause the hesitation problems.  If you run the engine at
night in total darkness and use a spray gun to spray water into the engine
compartment you will see a fabulous fireworks show of nice blue sparks all
over the place if your spark plug wires are hosed.  If you replaced all of
these items in the "standard tune-up things" then disregard.

With digifant motors the coolant sensor is vital to the ECU in responding to
fuel/timing/mixture requirements.  Check this for the hesitation problems
and cold running issues.  You will have to have a shop with a %C02 sensing
device to check the O2 sensor with the Bentley specs, which if hasn't been
changed in 100,000 miles is probably bad and causing all your problems.

Even though I don't own a vanagon YET, this has been my experience with
regular H20 VWs and Audis.

Rich Blake