It is probably not much to worry about. The weather has gotten cooler at
night recently, and metal and rubber shrink up. Perhaps a pushrod tube is
leaking a bit(8 rubber seals).It may be the rubber gasket on the oil
filter. Could be the drain plug for the oil, but this is all metal.
Anything other than the oil filter is most likely inside the engine tin
that covers each side of the underneath of the engine.If so, it has been
dripping for awhile most likely.It takes time for leaking oil to make its
to a place where it can drop onto the ground. Wish I had a oil free
engine.I got lots of drips from all possiable places it seems, and I keep
on trucking.I would look at the bottom of the oil filter(where you can
touch it)and look for a drop of oil on the filter case that is about to
drop onto the ground.A thightening of the filter  with 2 hands will do
it.If it is the oil drain dripping, a 10mm wrench will fix that.Take the
www.vanagonauts.com address w/you when you go. They have people that will
help you all over the country if you break down(vanagon rescue squad).Best
to go to Sears and get some screw drivers , metric wrenches ,ect.If you
break down on the side of the road, best to have some tools so if someone
stops to help you they may actually be able to fix what ever is wrong.Also
get AAA. I got this for my daughter,who is at college.She drives the 120
miles too often in an old car. AAA will tow you  for 100 miles 3
times /year.100 miles will get you to a vanagon friendly place anywhere in
the country. Cost about $85/year.Also get a can of fix a flat($5).