Unfortunately, trail and error and luck are how to find a great VA. Put systems in place which can simply be run through google drive or asana to assign tasks and keep track of work.
Upwork and Freelancer are two good places to find virtual assistants.
Maybe this could help you. They were recommended to me by a successful agent. So far, I've signed up to use their transaction coordination services only. Next step for me will likely be VA training etc. If you try it, let me know your thoughts.
All the Best, Joe Z