After a brief chat with Vans Support, it turns out the holes were drilled to the wrong size.
The fix is simply to drill them out to 1/4" and use AN4 hardware. I have the same problem with all of the other bolt holes for the seat ribs, so those will get drilled to 1/4" as well.
Vans is sending the hardware in the mail, so all is good again
One lesson learned from this is that in general, the design of these things is REALLY well thought out -- if something seems sloppy (like using an oversize hole and loading the bolt in tension for no apparent reason), it's well worth a call to Vans.
Luis