You do need a virtual switch, but if your locks are controlled by smart things, you don’t need IFTTT for that particular use case. You just use the big switch for hello home actions and have it unlock the door when you flip the virtual switch.
See number 31 under convenience:
You only need IFTTT in the mix if you are connecting to devices which are not controlled directly by the SmartThings.
Just remember with locks that there’s no security, this is a voice input. If you use “Alexa, turn on front door lock” to flip a virtual switch that unlocks your front door, someone could stand outside your house and yell the same thing and the door would unlock.
However, and this may well have been what you were thinking of, one of the big advantages to virtual switches is that you don’t have to assign actions to both the “on” and “off.”
You could set up a virtual switch that locks the door, and never set up anything for echo to use that unlocks the door. Then some stranger yelling could lock your door but they couldn’t unlock it. 