Vinasun are pretty reliable. There are some others that are good too - I like the yellow Vinasun taxis (looking more bashed up these days) and Mailinh group. Dodgy ones often have generic names or look-a-like logos. After a while you soon begin to recognise the reliable ones.

If coming out of a tourist spot, it's worth walking a block and picking up a taxi that's on the move. The dodgy ones often focus on well known tourist places. Any half decent restaurant/hotel/expat bar will happily call a taxi for you and don't hesitate to specify your favourite company.

No need to tip, but rounding up is usual. Taxi drivers love not to have small change. So split large notes in shops/bars etc so you don't run short.

Back to the airport - if you have VN dong cash and have your hotel/address written down, then no need to buy a coupon. Make sure they start the meter. You have to add the amount of the toll on exist (can't remember - VND5000? VND10000?). If you get into a dispute, just ask the hotel on arrival to deal with it.