Vast.ai is described as 'GPU Sharing Economy: One simple interface to find the best cloud GPU rentals. Reduce cloud compute costs by 3X to 5X' and is a Cloud Computing service in the online services category. There are more than 25 alternatives to Vast.ai for a variety of platforms, including Web-based, SaaS, Linux, Windows and Self-Hosted apps. The best Vast.ai alternative is DigitalOcean. It's not free, so if you're looking for a free alternative, you could try Elasticsearch or Amazon Web Services. Other great apps like Vast.ai are Runpod, Golem, Chroma DB and DedicatedCore.
Digital ocean doesn't rent out systems/instances with GPUs. The two different platforms serve different needs. DigitalOcean is great for VPS, web hosting, and webapps...but not for cloud computing machine learning and AI.
Vast.ai is awesome if you are doing anything machine learning or AI and need a lot of computational power. It is docker based and is very close to having a bare metal experience with top of the line hardware.