I've never seen or tried an S&S header for a Vanagon, but...

I did buy one for my 83 GTI a few years back.  I still have it and it hasn't
burnt through in the roughly 20miles I've had it on.  That's about all I can
say for it.  Oh yeah, it was cheap too.  Compared to a new manifold, which
was my alternative at the time.

Against it, I can say:

1) The tailpipe hookup tube (just downstream of the collector) is the same
exact diameter as the other 4 tubes, which is to say about 1/2" smaller in
diameter than the rest of my (factory) exhaust system.  Yet it was supposed
to be made for that car?

2) The flange on the hookup tube was welded in place about 3/4" too far
forward to attach to the rest of my exhaust.  Luckily I was able to cut it
loose on my lathe and have a friend reweld it in the right place.

3) It hangs low enough that with generous throttle applications the
collector hits the swaybar.  Even with fresh motor mounts.

4) As low as that is, it's still too high for a correct Bosch oxygen sensor
to fit because the boss the sensor mounts in is on the top if it, at the
lead edge of the collector.  On deceleration the sensor would whack the
exhast heat shield and broke the wire off within a week.  Like it was doing
a whole lot of good anyway, reading exhaust gas off primarily one single
cylinder's exhaust stream and being so far down stream (compared to the
original unit) that the gases it did see were much cooler than they ought to
be.  I managed to modify another sensor enough to get it to fit.

5) I also managed to make some custom heat shields to protect the
replacements for the plastic shift linkage bushings and the steering rack
boot that couldn't take the heat.

6) I noticed a -very- slight (might be my imagination even) power increase
in the midrange, but nothing and maybe even less than before, in the high
end, which is where I had expected an equal length tube header to shine.  I
attribute this to the small tube downstream of the collector.

7) I've seen several other cheap, but other brand, headers break welds at
the head flange from flexing there instead of at the joint where it was
supposed to, so I also had my welding friend attach a bracket I made right
near the rearward bend.  That bolts to the block and makes the flex thing do
the flexing instead of stressing the head flange welds.

Again, most of this likely won't apply to a Vanagon pipe, it's just my two
pfennigsworth, but it may be an indicator of the company's overall attention
to detail and quality.

Carl
82 Vanagon which really likes it's Dansk I got from Ron.
83 GTI
86 Scirocco 8v