But you CAN'T choose where to apply, it must be the place your physically located. If you were to travel to Vietnam (from Australia or NZ) just for it, you'll need to pay airfares and at least two weeks hotels, so you wouldn't save much.
the average price is around USD 400, but in each country has different exact prices. Australia and New Zealand are notorious for charging well over the average.
a visa run can be done by land or air, as the main purpose is to "renew" one's stay in or order to keep living without a visa. Hence, the term it's not applicable for DTV or other visa types visa holders. As for we're concerned, we can exit and enter freely by land or air as many times we want.