Upvote:6
There are no "ferries" that travel the route. Ferries are not available for this distance since the minimum distance (bee line) from Hong Kong to Vietnam is 600+ kilometers. You would rather get on board a cruise ship.
Here are some cruises departing from various places in Vietnam, this one here for example going from Singapore via Vietnam to Hong Kong.
As an alternative, and maybe more interesting, you can also take a train. However this will require you to get a Chinese visa, which the boat does not need.