Consider this scenario. The view and angle through the viewfinders for the following two scenarios will be identical:
Canon 7D with the 18-135mm zoomed to 18mm.
Canon 5D with an 18mm Full Frame only lens.
If you had no idea what camera I handed you, and gave you these two configurations, the image through the viewfinder would be identical and you wouldn't know which configuration I gave you.
In that scenario (EF-S lens at 18mm on APS-C vs. EF lens at 18mm on FF), the FF viewfinder (and captured image) would show a much wider angle of view.
If you are at 18mm on APS-C and 29mm on FF, then what you see in the VF will be the same.