Wow - I'm really appreciative of all of the responses I've been getting on this thread!

Before I get back to the itinerary edits, I just wanted to mention that we'd actually based our itinerary on the suggested 2-week itinerary in our Lonely Planet guidebook. We actually took stuff OUT of their list, if you can believe it, and added in Siem Reap with the "extra" days we'd saved (from their two-week plan, we scrapped Ninh Binh, Nha Trang and Mui Ne). Judging that this is their recommended itinerary, I was kind of surprised that everyone has been so universally and vehemently opposed to it! Anyway, I do appreciate your advice and recognize the value of staying in a place longer, I just wish LP hadn't gotten my hopes up about the feasibility (or at least enjoyability) of their itinerary!

Anyway, to answer a few of your questions:
- The night in Tokyo was booked with our original flights months ago, so that's not really negotiable at this point.
- Flights to Siem Reap are also already booked as per the original schedule, but they are changeable/cancelable for a small fee (Vietnam Airlines does fly direct to Siem Reap from Da Nang, so we are direct there and direct again to HCMC on the way back). I had actually just booked the flights the night before I started getting second thoughts and originally posted this thread. Before posting, I did check to confirm I even had the ability to change/cancel, which I do, so I thought if we did end up changing our plans, the fee would be (literally) a small price to pay for having the holiday we ultimately want.
- The Hue airport is already closed, which is part of why we intended to do the overnight train from Hanoi to Hue. Also, as thursdaysd mentioned, the flight schedule wouldn't work with the return from Ha Long Bay anyway, but we can get back in plenty of time to take the night train (it departs at 7pm, arrives Hue 8am).

As I said before, I'm up for cutting something out of the itinerary, but I don't know if I want to do JUST Hanoi, Siem Reap and HCMC - after all, I have seen Angkor Wat before and even in three days I would be seeing a lot of the same temples as I did the last time (last time I was there three nights/two full days, and, while still wanting, felt decently satisfied I'd hit the highlights). I would still love to return and I'd love for my husband to see Angkor Wat, don't get me wrong, but we also value the experiences in Vietnam that neither of us have done before. No one has yet touched the possibility that we not do Angkor Wat at all - are you all in agreement that this is worth anything we could give up in any part of Vietnam?

In that case, if we're going to keep Angkor Wat I'd still like to keep either the Delta or Hue/Hoi An. Which of those two would you recommend? The Delta didn't seem to get rave reviews - is it more enjoyable if you do an overnight, and just a waste if you only do a day trip? (Ha Long Bay seemed to get a similar rep in terms of not being worth it for only one day, a Vietnam highlight if you overnight.) If we did Hue and/or Hoi An, should we just stay in Hue? We're more interested in the sights around there than shopping in Hoi An, but it did sound lovely too. We arrive in Hue on the morning of Day 8 and could stay all the way until the morning of Day 12 and just go straight to the airport in Da Nang, or go to Hoi An on Day 11 (since it's so close to Da Nang), see just a bit and overnight there before going to the airport on Day 12. Or do the Delta??

Thanks again for your insight!!

I would definitely recommend Hue/Hoi An, or just Hue, over the Delta. Also keep in mind that it will be hotter in the south.

It is hard to say whether you should skip Siem Reap. If you expect to come back to SEA it would make sense to just do Vietnam this time, but Angkor is in a class by itself.

Since you've been to Angkor you know much better than anyone else how much time, if any, you want to spend there.

It will be hotter in Hue and Hoi An than it is in Saigon. The main difference is that it will be much wetter in the South than in Central Vietnam. The Central Vietnam coast gets very little rain in July.

Based on your Hue/Hoi An plans, I would stay 2 days in each. And if possible when traveling from Hue to Hoi An, take a car over the Hai Van Pass. Spectacular views.