On the plus side:

1  With a frame attached hitch the Vanagon is a quite stable tow vehicle.
2  Once your going there's actually not very much extra drag added by a boat
   behind (on the flats)

On the minus side:

To get going (ie accelerating or going uphill) the torque needed
from the engine is propotional to the combined weight. Getting started or
going uphill is thus going to require almost double the torque
for a 3500lbs Vanagon and 3000lbs boat compared to the 3500lbs
Vanagon alone.

Two solutions to this:
1 A tranny with a low granny gear (to get started)
2 Or a lower final drive ratio (Most trucks come with a choice
of 3 or 4 final drive ratios, and when you buy it you tailor the
final drive ratio to the weight you will tow.)

Note that when starting from a standstill, say to pull up the boat on
a boat ramp, the waste heat generated before the boat+ Vanagon combo
actually can move in an auto tranny and a manual clutched tranny is exactly
the same given equal gear ratios and engines.

An engine with more lower end torque will generate less waste heat
(= less wear on clutch or auto tranny) than an engine with less
low end torque.

/Martin