Recently I had to reset the root password to my vRA 7.3 appliance and I had to follow the VMware kb article https://kb.vmware.com/s/article/2150647
Only thing I would change in the kb article instructions would be the 6th step.
6th step — look for the work ‘vmlinuz’ in the second line and then hit e on that line to edit the line, then add the command init=/bin/bash to the end of the line and hit Enter
7th step — hit b to boot from that line to get to the root prompt
10th step — After you reset the password and it says that the new password has been accepted, type reboot to reboot the appliance
Hope this helps!