Hi,
The vSphere Client does not support adding custom top level tabs for vSphere objects (such as VM). However, it is possible to add such tabs for custom objects.
You can, however, add a category in one of the existing tabs (Monitor/Configure...). To learn more, please visit: https://code.vmware.com/web/sdk/65/web-client. (link to "vSphere Web Client SDK Programming Guide" at the bottom of the page ). Chapter 12 describes the list of currently supported extension points.
If you want to register your plugin to the Extension Manager, you can use the "vCenter plugin registration" script provided with the SDK zip (under "html-client-sdk/tools/")
Best Regards,
Denis