Why do we assume 100% ionization for acetic acid (a weak acid)?
You do not assume 100% ionization of acetic acid during titration. Acetic acid is a weak acid. Look at this equilibrium

$\ce{CH3COOH &lt;=&gt; H+   + CH3COO-}$

The base, $\ce{NaOH}$, keeps consuming $\ce{H+}$ during a titration, as a result acetic acids keeps on providing $\ce{H+}$ to maintain the equilibrium in water until all the acetic acid is converted into acetate ion.