Vast.ai seems like a scam to me. I funded my account with a little bit of money ($25) and tried repeatedly to reserve an instance, but it would just hang: no response, just waiting to provision it.

Gave up after 4 attempts and wrote the $25 off as a cost of the learning experience.

It works perfectly well as long as you realize some machines are simply broken.

For example, ones in China simply never can pull images. I don't know how they got approved!