vMA is a virtual appliance that has remote command line tools (vCLI) pre-installed and ready to use, when you set the target you are just saying which host to run the commands against not actually making a remote SSH connection to the ESXi host.
That CLI is only on the host and not part of vCLI and therefore not accessible via vMA.
We are also having a similar need to run native shell commands outside the realm of stock VMA. If you are able to utilize third party/community supported modules on your ESXi host, you might try this:
That should allow you to run what you need. Without that, it is not accessible.