Carry your passport with you everywhere if you travel in Vietnam. In every hotel you should show it on the reception before check in there.
My first trip was in Mui Ne last month, we were living in the room in a restaurant’s backyard and the guy didn’t ask me for my passport. This time when I went to Da Lat I forgot my passport again and had problems there.
First night we checked in to the hotel with my friends and Chinese girl. I didn’t have passport but the guy on the reception called to my condo in Nha Trang and they sent him pictures of my passport and visa.
Next night I didn’t check in anywhere until late evening. In the evening I with my new girlfriend was trying to check in to the hotel where my friends moved for second night. We agreed abut the price and guy from reception gave us a key from our room. But then he called us to room and asked me to come back to reception to show my documents which I didn’t have.
After some arguing we forced to pay for room and moved away. We went to another hotel where nobody asks for documents but unfortunately that hotel was fully packed. Then I decided to go to the hotel where I spend the first night. That hotel was also packed but that great guy who called to my condo before printed out my documents and with that paper we came back to our room, showed to that guy my paper and checked in finally.
Keep your passport beside you in Vietnam)