Viennese makes most people feel a little sick when you start out. Your instructor is right that spotting can help; however, this can quickly become dangerous, especially as a leader. The most obvious problem is that it is harder to keep track of where you are going, keeping an eye out for where the other dancers and avoiding them, if you are worried about spotting. Spotting is far better when you are doing spins in place rather than with a moving dance like Viennese. Another problem with spotting is that it will cause you to constantly be flipping your head around. Head weight is very important in dancing as your head weighs a great deal. Whenever you change its positioning, you change the balance in the partnership. Try dancing down the floor with a lady who keeps turning her head back and forth. The reason for dance position is that you are each counter-balancing each other. If one of you were to start shifting around, it would thrown the balance off.
I would agree that it is best to start out slowly and let speed come with time. Also, keep in mind that it is meant to be a linear dance. The point of it is to get you down the length of the floor, not spin in tiny circles all the way down. Also, start with American style Viennese. This is slower than international which will give you more time to think/dance.