FYI, there's a WIP PR for Vultr backend support in NixOps.

https://github.com/NixOS/nixops/pull/634

I cherry-picked this into my local NixOps repo and have been using it for several weeks to deploy to Vultr servers with no issues, except one -- the author has hard-wired an assumption that you're using Btrfs for your root filesystem. I didn't want that so I simply replaced that hard-wired assumption with ext4 in my personal branch. Now everything is working great.