US Vanagons have fuel injection, so most of us have never seen one with a mechanical fuel pump, but here's my guess, based on experience with older cars with mechanical pumps:

The diaphragm is an internal part of the fuel pump.  You can probably fix the external leak by changing the seal (or tightening the bolts!), but if the pump is cheap it is best to replace it so you don't have to worry about the condition of the diaphragm in it.  If the diaphragm fails completely, the car stops running, but if it just leaks a bit you get fuel in the oil which degrades the lubrication ability of the oil and can destroy the engine. (or the leaking fuel catches fire and destroys the entire van!)

Bill

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 I got this advice from a guy called Jim at the Passion forum at
 www.mtbr.com (he might also be around here)regarding this image:
 http://superelectric.net/transporter/0004.html This is his advice:

 "Looks like the mech fuel pump is leaking at the base. I'd go ahead and
 replace the pump (super cheap) along with the seal, if it was mine.
 Check and see if it's a oily/gas mixture, if so, the diaphragm is
 leaking. I've ruined a perfectly good 1600cc in a beetle with a leaky
 diaphragm."

 I guess changing the pump is a fairly easy (is it really needed,
 should'nt it be enough to change the gasket?), but I have no idea on how
 to change the diaphragm. Any online descriptions on the procedure? And
 how expensive is a new fuel-pump?