How can I measure VPS performance?

Hello I had a similar task and I've done the following(if the server is a windows VPS ) :

1. Define what is more important for your application/enviroment (for a DB Prod server you might expect a high read speed , CPU ) . You can get that from the aplications \ event log and see which seems to be the bottleneck.
2. Perfmon counters for the things that you've decided to monitor at step 1 . Be sure to make them detailed enough to get some good data but not get them every minute (usually @30 minutes should give you an general idea )

3. If you have an MSSQL instance there be sure to monitor it's performance and CPU \ RAM limits .

4. If you have IIS be sure to check the logs and if any application has "fatal" errors

5. Check IIS logs and get performance counters .
6. Check Event logs for any errors , usually fixing them should give you an idea and will give a small boost to your VPS .

For Linux VPS :

1. top -s should give you an basic idea of the process and which one consumes most of the memory
2. to test read/ write speed try to copy something and see the speed . For disk access time , you could run a MySQL query and see the execution time .(and most important which operation takes the most in your query )
3. see dmesg for any error messages and try to fix them .
4. if applicable you could try to add it to a Nagios monitoring server .(there you can also find scripts to help you monitor )
5. see the logs of your applications and if they report any performance / issues .