After my second 30 min try on the horn with various Delta reps, I finally managed to reach someone who knew what he was talking about.

Here's the current situation with GoAsia pass. No restriction on gateway cities other than your partner has to fly there, so I can use BKK for Vietnam Air but have decided to try to get to Hanoi with Korean Air first. You must buy at least three coupons and the first coupon must be used to reserve a flight at the time of coupon purchase; other legs can be booked any time after purchase but within six months after the first coupon flight. Coupon prices depend on mileage: 0-200 for $90, 201-600 for $130, 601-1000 for $170. I checked these prices vs web fares and I would save about 30% by using coupons for my itinerary.

So, the coupon savings seem to be in the neighborhood of what SOOROX was talking about for an in-country travel agent. Because my itinerary is completely on Vietnam Air and it is pretty well fixed, I may just reserve all the legs when I buy the coupons - that should eliminate any problems of acceptance since in theory I would have a ticket instead of a coupon (I hope).