I've never had an issue, but I've seen/heard of instances, where a tip got people through immigration without a visa or was paid in lieu of duties at customs.

IMHO, this is not all that different than the developed countries. It's just that they have passed laws in the developed countries to legitimatize the process. It's still a shake down either way. The only difference is that in places like the U.S., the CBP will pull their guns and then lock you up in cage if you don't pay.

Agree it's more of a problem for Viet Kieu.