What is the difference between virtual queue and callback?
There is no difference--virtual queuing and the callback option are the same feature in Bright Pattern Contact Center. For more on virtual queuing, refer to the Virtual Queue Tutorial.
For callbacks, we utilize a method that adds callers to a list, which is then processed by a separate service. Is there a way to monitor how many calls are pending in these lists?
This should be as simple as monitoring the real time metric total_remaining. To make a quick distinction, you can see how many calls can be dialed right now by looking at dialable numbers, whereas you can see how many callbacks are pending a final disposition by looking at total_remaining.
For more information, see the Supervisor Guide, section General Information About Campaign Operation.