Virtual Device is a construct which is part of the official SmartThings platform. We used to use them a lot more before Core and webcore, but they still have their uses now.
Virtual switch is a device which only exists virtually. There is no physical switch. But it does have a device type handler, and the smartthings platform will treat it just like all of your other switches. It will show up in your things list, you can subscribe to it going on and off with a routine or with webcore or the official smart lighting feature or any smart app.
So within the cloud, it looks like a switch, it acts like a switch. It’s just that there is no physical switch.
So, you can make one of these whenever you want and you add them to your account through the IDE just by doing add a new device and choosing one of the virtual device device types for that new device.
You can’t add it through the mobile app because it can’t be discovered in the physical world. But that won’t make any difference – – once you have added it to your account, as I said, it will show up on your things list. 
Here’s the FAQ