After reading countless nightmare stories (including from Nha Trang) ending with "I should have never let them hold my passport", I always keep my passport with me while traveling. Some Vietnamese hotels initially have asked to hold it, but upon my request, all have accepted a photocopy, photo, and/or a cash/credit card deposit.
All excpet Le Soleil Hotel. During check-in, staff said he would "hold" my passport". He refused my alternative suggestions, argued belligerently, and gesticulated with my passport in hand as if playing a game of "keep away". Only when I stared him down and ordered him to "give me my passport back, right now!", then he finally relented.
To confirm, I asked "to be clear, you are saying I cannot stay here unless you keep my passport?" The staff said "yes". I walked out the door.
Something was wrong with this man. His tone during the interaction was more like someone who might be associated with the criminal underworld versus any hotel employee I have ever met in the dozens of places of all types in which I have stayed in Vietnam.
I then had to wander around Nha Trang after midnight searching for new accommodation. Due to high season, it took three hours to find another place. This unsafe situation was directly caused by the negligence of Le Soleil Hotel.
Furthermore, the hotel has thus far (five days hence) stymied my attempts to obtain a refund by dragging out the process with Hotels.com customer service. Be warned!!!