To monitor bandwidth usage of each service individually, you will need to install third party bandwidth monitoring tools in your VPS.
To monitor bandwidth usage in Linux VPS machine, you can install following command line monitoring tools. These commands are very useful to monitor network and find bandwidth bottlenecks.
You can use the command ifconfig to see what is the total amount of data that has been sent and received by the network card.
If what you need is to monitor the server from a remote location, then that is something completely different and there are i assume thousands of articles about that.
You can also use some server monitoring tool for that. Some of the most popular ones are Cacti and Nagios. They can be easily installed on a Linux server. Otherwise, like you mentioned, you can simply use the commands to monitor the servers.