I just got a set of 15" wheels from Ken Wilfy at Vanagain. they  are 15x6.
I mounted a set of Falken ZE-502 225/60R15 rubber, which is a square
profile, aggressive street tread tire...with an H speed rating.
What you will find is that if you are looking for a "street" look as
opposed to a truck or van look, then you have to be willing to consider
some tradeoffs. The standard tire for a Vanagon requires a "C" load rating
or at least a load rating of 1620 pounds per corner. Getting this kind of a
load rating in a shorter sidewall tire is difficult to do. Generally
speaking, the shorter the sidewall, the lower the load rating. The tires
that I chose have a load rating of 1520 per tire. I am trading some weight
bearing capacity for the "look" I wanted. In my case, this works because
the van is a weekend warrior and it is unusual for it to carry more than 2
people.
In terms of performance, the shorter, stiffer sidewall improved the
handling and the ride of the van. Steering is quicker and body roll on
corners is reduced because of the shorter, stiffer sidewall. Another area
of tradeoff with this kind of tire is going to be longevity. I suspect
these tires will only last 30,000 miles or so. Since my rig isn't driven
every day, this was a tradeoff I could live with.
Because the wheel size increased an inch BUT the overall tire diameter
decreased 1-2 inches, my transmission needs no changes and I suspect that
my speedo is within 5MPH from where it was with the old tires.