Last time I checked Vultr included a DDoS service [1] an DO did not.
Caveat: I don't really know much about this service, maybe someone here can chime in?
* I don't know how reliable it is
* I don't know if a small/medium app really needs it
* I don't know if you would be better off using something
like cloudflare anyway
* Etc...
What I do know is that a long time ago I used to host some stuff on a VPS (Linode I think) and I would routinely run out of bandwidth because of traffic coming most of the time from random AWS IPs, which seemed like shady bot networks or something. Ergo having a DDoS service like this seems useful even if you are hosting a small site.
They have POPs in parts of the world DO doesn't have. E.g. they have a Sydney POP which attracts customers from Australia and New Zealand. Location was pretty much the main reason why I chose Vultr over DO as I found everything else was more or less on par.
Vultr allow running on bare metal and they also allow access to BGP. That's both niche needs, but that's also what you expect from a smaller player, to fill niche market.