Hey Richard,
I am brand new to wholesaling but I can speak based on what I've learned on Bigger Pockets, mind you I am still learning also. About the property being vacant, no it does not have to be vacant for you to wholesale the property. You'll run into owners of rental properties who are just tired of the property and want to sell but do not want to evict the tenant. They would sell the property but tenant stays. You would have to find a buyer who's willing to buy the property with the tenant. The buyer would then decided what they would like to do. Depending on the lease agreement the tenant has, it's possible that the rent amount may increase or the new owner may give the tenant a time period to vacant the property.
About the mortgage part, a lot of wholesalers go after homes that are in the process of being foreclosed so I would think that having a mortgage left in it wouldn't be a problem when it comes to buying and selling.
Please correct me if I'm wrong fellow BPs. Like I said, I'm still learning. BTW, I was born in Newark, NJ Richard!