Not quite aligned with mail order brides, but here's my experience with Vietnamese prostitution in Ho Chi Minh city last week.

Stayed in what some call the 'backpacking district' of Ho Chi Minh city. While prostitution may be illegal, I can't count how many times I was offered a massage by girls on scooters walking out of the higher end clubs. Also, they line the walls inside certain clubs like the Raging Bull and CO2 waiting to be picked up by drunk men as they go to leave. My buddies and I went to a massage parlor and were offered happy endings near the end (we declined for a variety of reasons). Old dudes approached us on the street during all hours of the day offering girls or marijuana.

We came to realize that just about every massage parlor/spa/beauty salon has a girl out front handing out similar fliers detailing the wide variety of different packages they offer, but upon entering they don't understand any English anyway; they just do a little bit of everything and offer to jerk you off at the end for a tip. Down the street from where I stayed there was a hole-in-the-wall hotel with pink neon lights that only seemed to look open at night. As you walked by scandalously dressed young Vietnamese girls would call after you inviting you in for a "drink and massage".

I noticed very little police presence in that area, just private security for every business. When we did notice police they were usually together hanging out near the intersections watching the cluster fuck of traffic passing through the roundabouts. Besides that, I never saw a vehicle pulled over, anyone being questioned anywhere, or any other sorts of police interactions.