To use "viewpoint" to describe a personal opinion would not be used in US English. We would use phrases like, From your point of view, what is .. What is your view on ... "Viewpoint" has just the second meaning mentioned by SuKi, of a physical location

If you are talking about a personal opinion, they have the same meaning. 'Viewpoint' can also be a physical location, such as a place on a mountaintop from where you can see a panoramic view.