It’s reasonable safe to travel with Soft Sleeper Class on Vietnam Railways. I read that the main consent was the belongings will get stole. But not in our case. In our carriage, there was a Vietnam Railways warden who slept on the carriage herself. Once the train departed from Ga Hanoi train station, she knocked each cabin, and demanded us to stored our luggage below the lower berth, and kept everything valuable under the pillow.
What she did next was to lock all the doors to the carriage! This effectively disallowed all strangers and possible thiefs to get into the carriage. Although at other carriages, they didn’t seem to lock the doors. And from safety point of view, it might be a nightmare if anything happens.