To some philosophers, this statement is absolutely true. They argue that all vices tend to enslave people. They say that when we indulge our vices, we are letting our desires control us and are not truly free. In order to speak on this, you should introduce this idea to your listeners and then give examples that tend to support it.
The clearest examples of how this is true come from various sorts of addictions or compulsions. Smoking is one example. Many people who smoke are addicted to nicotine and can therefore not stop smoking. They lack true freedom because they cannot decide for themselves (not easily, at least) whether to smoke or not. For some people, overeating is the same way. They know they shouldn't overeat, but have a very hard time abstaining. This, too, seems like a lack of freedom.
So, to do this speech, introduce the idea that indulging in vices allows your (generally physical) desires to control you. It is only if you get rid of vices that your free will is in control. This is the essence of freedom since you are able to choose what you do. Then give examples that you can think of that support the idea.