My very first trip to Vietnam, I was in the immigration line and the German guy ahead of me was trying to explain that he couldn't give the name of the hotel, because he had hired a guide, who was waiting just outside, and the guide had booked the hotel and not told him the name. The gruff uniformed agent listened with barely concealed contempt and boredom and then told the German, "Go stand over there."
I was next and I was screwed, because I was in the exact same spot, I had hired a guide and not asked what hotel I was staying in -- and if I tried the same thing, the agent would think I was just fucking with him and probably, I don't know, deport me or arrest me or shoot me. Something.
I got up to the desk and the agent grunted, "Hotel?"
"Hotel Orient" I piped up, a name I invented on the spot, using the flimsy reasoning that we were in the Orient, and a big city like Saigon, there probably was at least one Hotel Orient.
It turned out that my guide had in fact booked me into the Hotel Orient.
Since then, on every entry form that requires a local address, in Vietnam or not, in Asia or not, I write "Hotel Orient".
No one has ever questioned it, or even noticed.