@Anthony ****
That's one part of it, but some others mention including a photo of their passport, showing the entry stamp to the country, where the application is being made.
If, for the sake of argument, one booked a hotel in Hanoi and used that booking as evidence, whilst they're actually applying from Thailand, it might work. What if they ask for more evidence, such as a photo of the entry stamp to Vietnam?
I'm not saying it's likely, but it's certainly possible. With the e-visa process, if your application is rejected, you lose the fee.