I saw acetic acid being used in two reactions:

I was asked to predict the outcome of the first reaction but I couldn't, because nitration requires generation of $\ce{NO2+}$ which in turn requires an acid stronger than nitric acid. (acetic acid is weaker)

I would like to know how exactly is $\ce{AcOH}$ participating in these two reactions. Is it supporting the generation of electrophile? If yes, then how? I couldn't find any relevant information on the net. The second reaction is from Clayden's book and it doesn't highlight the role of $\ce{AcOH}$ there.