I just took the slow boat (from Phnom Penh, Cambodia to Chau Doc, Vietnam a week ago, and was very displeased to find that Vietnam's Sinh Cafe seems to have a monopoly on the service. It was OK, but a bit of a gimmick as it involves a 2-hour bus ride (1-hr around town and then 1-hr to the boat), which seems to be the main time difference between the slow boat and the express boat times.
I did realize a scam between Sinh Cafe and the Vietnam Immigration office however--and I wonder whether the express boat customers have to put up with the same thing.
When you get to the Vietnam immigration office, instead of waiting in the building, the immigration officers march you out back to a restaurant and tell you that you have to wait there for an hour while your passports are processed. They then of course demand you must order something. All of us at the time refused and marched back into the office and waited there.
The next portion of the scam was a "passport processing fee" of 2000VND. Tiny fee, albeit probably a fake one. Obviously, having just arrived from Phnom Penh, no one (except us expats) will have VND in our pockets--so the Sinh Cafe employees demand 1r Cambodian instead, on behalf of the immigration officers. Of course, 1r is worth more than 4000VND. THus, the Sinh Cafe employee pockets 2000VND, as does the immigration officer. CHump change, except that when you tally up all the tourists moving through the border, then the Sinh Cafe man and the Border guard can each make a whole week's salary in a single day from this little fraud.