Well, then, you have only one chance, maybe, but only if there's a US-based carrier that flies all the way to Vietnam. Is there?

Here's the reason why: With all US-based legacy carriers, you can get the "last" seat on any plane they fly by using only double miles. You may think that's a lot, but compare that to those point programs that everyone talks about: They charge more and more and more points as the cash price of the fare goes up, with no limit (and often going way beyond double for "last" seat points awards).

But no carrier that I know of has these double-miles-for-the-last-seat awards on partners, only on their own metal.

Which is why you need a carrier to get you all the way to Vietnam on their own metal, who has a double-tier structure where the top tier means you can get the "last" seat.

I said US-based carriers, because those are the only ones I'm aware of that have double miles for "last" seat awards. But if there are any foreign carriers that also have that, then I guess you could look into those too. But again, you'd have to fly that carrier's metal all the way from home to Vietnam for this to work.

Again, your only other option I see possible is booking 11 months out. Since you know the date and destination every year, is that possible? (The idea is that at least sometimes on some carriers, the moment they open seats at 330-360 days out, that's when they are most likely to make a couple seats avialable as awards. Though of course nothing's guaranteed. And you may need a subscription to a tool like www.expertflyer.com to monitor that...)