Is your Linux server accessed by multiple users? If so, consider adding an SSH login notification whenever someone logs in.
The examples below send an email when someone logs into your server. For this to work, your server must be able to send mail using the mail command. You may follow this tutorial to setup Exim with Gmail if your VULTR account is not verified to send mail.
Replace YOUR_EMAIL_ADDRESS with the email address that you want to receive login notifications.
CentOS
Open the file ~/.bash_profile in a text editor.
Append the following lines:
IP="$(echo $SSH_CONNECTION | cut -d " " -f 1)"
HOSTNAME=$(hostname)
NOW=$(date +"%e %b %Y, %a %r")
echo 'Someone from '$IP' logged into '$HOSTNAME' on '$NOW'.' | mail -s 'SSH Login Notification' YOUR_EMAIL_ADDRESS