In order to connect your Vultr account with Mist.io, you will need an API Key. You can find the API key on the settings section on Vultr. Login to https://my.vultr.com/ and on the top right select settings. There make sure the API is enabled and copy it's value.
On mist.io, press 'Add cloud' and select Vultr from the drop list. Enter the API key and press Add.
You should now be able to see your Vultr machines, neatly tagged for easy access. By selecting a machine you can run actions (reboot, stop, destroy). To provision new VMs, click on the Create button on the Machines section. By providing an ssh key on the Keys section, you will be able to connect through a web console, and enable monitoring to be able to see performance diagrams and set alerts and automated actions.