How will the presence of hydrobromic acid affect the percent ionization of acetic acid?
Acetic acid is CH3COOH. Let's save some typing by calling acetic acid HAc (The H from the COOH part and Ac for the CH3COO^- part).
HAc ==> H^+ + Ac^-
HBr ==> H^+ + Br^-
You know HBr is a strong acid; i.e., it is 100% ionized. Therefore, you are adding excess H^+ to the acetic acid (that is a common ion) and it will do what you think it will do; i.e., by LeChatelier's principle, the ionization of HAc will shift to the left which means less ionization and a smaller percent ionization.