Short answer – yes.
You can absolutely use a standard monitor in portrait mode which essentially turns it into a vertical monitor. However, the catch here is that the monitor should have an adjustable stand (or you can get one separately) that allows you to position it upright.
Dedicated vertical monitors come with ergonomic stands where all that you have to do to achieve the vertical configuration is rotate the monitor 90 degrees – without having to detach it from the stand and then reattach it.