I enjoyed myself watching it.
But... (yeah, you knew that was coming)
There were several parts where I actually became rather irritated at the show. The running gags about the gifts got old very quickly - yet they kept going on about it. I hate to think about the gag gifts that they referenced during Jezza's accident but never showed. Admittedly, it was probably the galleon that really went over the top - by the halfway point in the episode, I wanted to reach through the screen, grab that boat, and stomp it into a million tiny little bits. The bit with the boat and the ceiling fan was as scripted as something on America's Funniest Home Videos - and equally as funny.
The same holds true with the USA bike. Somewhat funny once or twice, the repeated references to it got quite old... plus I have to imagine it wasn't all that funny (ever) to any of the Vietnamese within eye or earshot, particularly those like the nice gentleman on the beach who lost his hearing in a bomb blast dropped from a B-52.
The scenery in this county was quite beautiful, and the Vietnamese people seemed to be gracious and kind. I know that Top Gear isn't a travelogue show, but still everything they showed along those lines made me want to see more. I know from reading other stories online, that Vietnam also has some incredible and amazing engineers and craftsmen... I'd have loved to see a challenge where they are to work with a team of Vietnamese engineers to overnight improve on the performance of their bikes (and a serious improvement, not a "stick twinkly things on it" type of joke bit that they sometimes do).
Which, of course, would naturally lead to the other thing the episode was missing: Communist Stig. They could've had a performance challenge, then they would "hand the bikes over to our tame racing driver's communist cousin." Had they taken out 90% of the practical jokes, the gag gifts, and the USA bike, they probably would've had plenty of time to fit this in.
So, with all those drawbacks... best I can muster is an 8. Still good TV, but just OK for Top Gear, and somewhat lacking for a Top Gear Special.
(The one question that I'm wondering - seeing Clarkson reading the menus and stuff, then having him answer the question in class, I'm wondering if he actually was able to learn some Vietnamese. That's pretty impressive if it was as it seemed.)