Question

What is the difference between a switch's physical interface and the vlan interface?

Answer 1

The main difference between the two is that VLAN interfaces are created virtually and can assign an IP address for remote access. They also do not have physical ports on the switch and are always up. Physical interface on the other hand can be connected to end devices. 

Answer 2

Switch's physical interface is something as the name itself suggests itself that it is a physical port to which you can plug in a cable or a wire based on its function and usage where as VLAN interface is no physical port but are created virtually when needed.