Same error can be found in these threads:
https://discourse.roots.io/t/provisioning-a-new-digital-ocean-droplet/7871/16?u=benword
I edited your OP so that it’s more readable, and not sure if the vault file you provided is exactly the same as what is on your machine (the salts aren’t aligned properly). Try running a YAML validator on the contents of it to make sure it’s good.