(This is Shaun writing.)
Surgery went very well, no problems! One of the primary risks of VSD repair is damage to the heart's electrical system (which then requires the installation of a pacemaker), but this was successfully avoided! No pacemaker! We are very thankful (and relieved).
Willem is in ICU now, and hooked up to every machine you can imagine. He should be off the ventilator in a couple of hours and should become conscious again shortly thereafter. He's still very much in critical condition, with much attention required, but doing very well.
Thanks everybody for your comments, gifts, thoughts and prayers!