what is the capital of vietnam

The desert you're looking for is the Kalahari. It covers a huge chunk of southern africa, mainly spread across botswana, namibia, and south africa. I've sailed past those coastlines quite a few times and the landscape is pretty distinct even from the sea. It’s not your typical sandy dunes everywhere though, it’s actually a semi-desert with plenty of scrubland and grass depending on the season.

There is also the Namib desert, which runs along the atlantic coast of namibia. That one is a lot more famous for those massive, towering red sand dunes that meet the ocean. If you ever get the chance to see it in person, the contrast between the dry desert and the cold water is wild. Most people mix them up, but the kalahari is the massive basin in the interior while the namib is that thin, sandy strip on the coast.