The thing to remember when buying automotive batteries is
that there are less than 10 actual battery manufacturers in
the USA.  These include ...

Johnson Controls
Delco
Douglas
Exide
Optima

The rest is all just labeling and marketing.  Probably even
the dealer's Autobahn batteries are made locally by one of
these, since lead is pretty heavy and low value to stand the
shipping charges from Europe.  Interstate is a label.  Best
bet is probably to compare specs (if there are any
differences available in the unique Vanagon size) and
patronize the supplier you feel will give best service.
Your local independent VW shop?  Another idea is to find a
battery specialty company--they may know how to get
batteries without the expensive brand name that are the same
thing.  I did this once with my late lamented BMW 6
series-BMW dealer wanted $150; same thing (exactly except
for labeling) at specialty shop was $40 or $50.