Looking at the map the beaches of Vietnam are nearer to Udon Thani than any Thai beach. Has anybody taken a Thai car through Laos to somewhere like Vihn? Is it doable?

Without any deeper research.
From hear/say, TV documentaries.

Close to impossible. Big hurdles.

If only I could drive to the beaches of Vietnam!

I live about the same distance from these as to the Gulf of Thailand and the Indian ocean.

One of the disadvantages compared to Europe where you can drive to almost all directions/countries whenever you like.

Just flying to Vietnam becomes easier and easier, more and more flights.

Just noticed that this is not the best sub forum for the question.

Have a look at this thread from 2015 in the motoring forum:

http://www.thaivisa.com/forum/topic/838833-driving-your-own-car-from-thailand-to-vietnam/

There is some dispute whether they now accept international driving permits or still insist on Vietnames driving license.

But that seems only one of misc. problems like right-hand driven cars not allowed.