Site24x7 monitors VMware resources using On-Premise Poller. On-Premise Poller is a lightweight component which you need to install in your network. Once installed, it will collect data according to the set poll interval and send that information back to the Site24x7 data collector.
You can add your VM as a monitor in Site24x7 and start monitoring. This help document should help you better.
Feel free to get back to us for further clarifications.