Daawgon--Thanks so much for the suggestion; I hadn't really been thinking much of Quy Nhon (but it sounds ideal--and likely much more convenient than heading all the way to Phu Quoc). How feasible is this plan?:
Nights 1-4: Hoi An
Nights 5-6: Hue (car or train from Hue An the first day; maybe DMZ tour on second day?)
Nights 7-8: Quy Nhon (earliest I think we can arrive on first day, flying through SGN, is 3pm. Would depart 3:45pm, two days later)
Night 9: SGN (unfortunately our departure flight from Vietnam is 11:30 the following morning, and connections that morning just won't work)