The 86-91 Vanagon uses a proprietary headlamp setup, so there is no direct
fit commercially available replacement.

The headlight plug is a 9004 type that will be available at most autoparts
stores.  The design of the 9004 bulb traps lots of heat at the connector,
which is a major reason that the plugs fail on a regular basis.  The best
9004 replacement plugs I have found are the ceramic plugs from
www.headlightplugs.com.  For small orders, they also sell on eBay
http://www.ebay.com/itm/9004-9007-Connector-Plug-with-14-GAUGE-USA-WIRE-/330391133304?pt=Motors_Car_Truck_Parts_Accessories&hash=item4cecd89c78&vxp=mtr .
I would recommend installing them using heat shrink butt connectors for best
results.  The pinout for the wires is available on my website here:
https://sites.google.com/site/vanagonheadlightrelays/helpful-information/lens-bulb-upgrades/headlight-socket-pinouts.

If your bulbs are more than a couple years old, it is probably a good idea
to replace them while you have things apart.  The best stock wattage 9004
bulbs I have found are the Philips X-Treme Vision 9004.  They are not cheap,
but they are the best you can do for stock wattage bulbs
http://www.amazon.com/Philips-9004XVS2-X-Treme-Vision-Headlight/dp/B00BQWQLOK/ref=au_as_r?ie=UTF8&Make=Volkswagen%7C74&Model=Vanagon%7C963&Year=1987%7C1987&carId=001&n=15684181&newCar=1&s=automotive&vehicleType=automotive

Another important aspect of better and brighter lighting, especially with
the 1986-1991 stock DOT lighting is to ensure that the inside of the
headlight lenses is immaculately clean and free of haze. Even the best bulbs
inside a dirty / hazy lens will only spread badly focused light and glare on
the road in front of you. Since the 9004 bulb is typically considered to be
one of the worst automotive bulb designs due to the transverse filaments, it
is worth going the extra mile to get the best results possible from them.

To do this, I usually use a bent nylon bottle brush with some all purpose
cleaner like Fantastic or Simple Green diluted 50/50 with distilled water.
Pour in a few ounces of the cleaner and swish it around for a minute, then
gently brush the inner lens surface, while being careful not to scratch the
mirrored reflector on the rear of the headlight housing.
After everything looks good, dump out the cleaner and rinse with hot water a
few times, then set aside to drain.

For the final rinse, I use a few ounces of 99% isopropyl alcohol, which can
be purchased at any pharmacy. Pour in a few ounces and swish it around the
lens and reflector to avoid water spots after drying. Then set the lens
outside in the sun for a couple hours until they are totally dry.

Once everything is back together in the van, be sure to verify the aim of
your headlamps to make sure you are getting the light where it needs to be.
There is a good procedure listed here:
http://www.danielsternlighting.com/tech/aim/aim.html.