Weak acids?

What occurs (if I'm remembering my chemistry correctly) is that your OH ions from your NaOH will neutralise free protons in the solution. After that occurs, the acetic acid is also free to continue dissociating in the water, since the proton concentration has changed. Acetic acid will exist in x concentration in solution but only y% (which should be constant) of it will dissociate and add free protons.

You should understand more as you approach titration and learn of equilibrium constants etc.