The Ecu is only connected to the battery via a relay, so with the key off, there is an open circuit and no danger of current bleeding into the ECU.Also, when welding you are not energizing circuits on the vehicle, otherwise lights would go on, the honk, etc.
The body of the vehicle is the ground heat-dissipating circuit and the positive is the rod or welding wire.
All that electrical energy is transformed into intense heat that welds the rod/wire to the welded part.You are not pumping electricity into the Vanagon when welding.
If you connected a jumper cable to a battery , and the other pos-neg ends of the cable to the vehicle, the cables themselves would melt , directly absorbing the energy from the battery.
That's what the wire or rod does, it meltsThat's my understanding anyway and I'm sticking to it til my ECU fries.Robert1982 Westfalia

      From: Steve Williams <steve@WILLIAMSITCONSULTING.COM>
 To: vanagon@GERRY.VANAGON.COM
 Sent: Tuesday, September 1, 2015 9:26 PM
 Subject: Welding on Van - ECU consequences?

Hi,

I am taking my van in tomorrow for a bit of patching on my exhaust system.

It suddenly occurred to me that they may be using an electric welder on
my vehicle rather than a gas welder.

Is it recommended/mandatory to unplug the ECU if electric
(arc/mig/tig/etc) welding is being done on a 1991 Westfalia?

Thanks,
Steve W.