The safest way to park is to park in a parking lot with a ticket.
Alternatively, several shops have a security guard that will guide you into parking. In some cases, shops do not have a security guard nor clear parking space. We recommend keeping the bike in a location where you can keep an eye on it.
In any case, do not leave the bike unattended on the streets, in Vietnam bikes not properly parked get stolen easily.
Be mindful that locals frequently leave bikes in some street areas, that is illegal parking and easily subject to theft or police confiscation.