V’al Hanissim is recited even when bentching after dark, as long as the meal began before nightfall and one has not yet davened Maariv. [For this reason, it is preferable to bentch before davening Maariv.]
On the night following Purim, one must not forget to daven Maariv as soon as possible. In any case, one should certainly daven before midnight (11:45pm).