I think this horrific crime is going to be solved by focusing on the "why".
Why was he there? (Appears obvious, but all possibilities should be considered)
Why would someone do this to him?
Of course we don't know what info LE has already pieced together, but I would think his communication records (phone, email, VM, Skype, chat, etc.) will piece the story together, even if he has been using secure transmissions
Here is a completely wild card theory; emphasis on wild card.
We now know the Perp is female, according to LE. (Yes, I ate my hat). :\
If the Perp wasn't someone he was receiving "service" from, what would motivate them to murder him? A few theories on the "why"
1. She was paid to do so by someone, perhaps over money or a business deal?
2. (and here is the wild card theory...)She was the *mother* of someone he had sex with, perhaps a minor?
I know #2 is a real wild card theory, but I am trying to think of reasons a woman who was not his wife, would be angry enough to murder him. In summary, here's what I've got so far:
-He didn't leave the wife as he had promised?
-The Perp was paid by a business rival/someone he owed large amounts of money to?
-A mother who was avenging an act that took place with her child?
-A member of the wife's family (honor killing) - yes, that is a big stretch and no reason to believe this, but throwing out all possibilities that seem within the realm of possibility. Vietnamese is a male dominated culture, no reason to believe that a man cheating would be cause for honor killing
That's what I can come up with at the moment. All just wild guesses, but trying to stay focused on the "why" and work backwards.