Use APKPure App
Get BirdCP old version APK for Android
View and manage your Vultr account and servers
View and manage your Vultr account and servers.
Find your API key on https://my.vultr.com/settings/#settingsapi
Also, allow your IP address here, otherwise the app has no access to your data.
I suggest you whitelist your home IP or all IPs to make sure you can access you servers anywhere.
DISCLAIMER
This is an unofficial app. I am in no way connected to Vultr. Vultr names and logos are owned by Vultr.