I have no problem here to run:
path\to\my\portable-VBox\directory\VBoxManage list vms --long
but as I find that rather annoying I typically first change into that relevant directory so that I can execute:
VBoxManage list vms --long
What I found important to note is that the VirtualBox COM server needs to be running at that time, as you otherwise get something like:
VBoxManage.exe: error: Failed to create the VirtualBox object!
VBoxManage.exe: error: Code REGDB_E_CLASSNOTREG (0x80040154) - Class not registered (extended info not available)
VBoxManage.exe: error: Most likely, the VirtualBox COM server is not running or failed to start.
You can ensure this by starting the portable VirtualBox application as this ensures that the required drivers are temporarily installed and the required services are running.