V’al Hanissim is recited in each Amidah of Purim.
If one forgot V’al Hanissim at the appropriate place in the Amidah, he may still recite it if he did not yet say Hashem’s name at the end of that Brachah. After that point, he does not recite it.
If one forgot V’al Hanissim at the appropriate place in Bentching, he may still recite it if he did not yet say Hashem’s name at the end of that Brachah. After that point, he should recite it as a Harachamon, as per the Nusach printed in many Siddurim and Birkonim.