There seems to be different Nuschaos regarding Al Hanisim. Some say Al and some say V'Al. What is the difference and what do most Halachic authorities recommend.
"Al Hanissim" seems to be the older version (as found, for example, in Machzor Vitry and in various siddurim printed in the 16th and 17th centuries).
R' Zalman Hanau (Shaarei Tefillah, sec. 110) cites and agrees with an opinion that it should correctly be with a vav, though, since it's a continuation of the preceding list of things we thank Hashem for. Mishnah Berurah (682:1) concurs.
On the other hand, R' Yaakov Emden (Luach Eresh, sec. 175) defends the older version, on the grounds that in Hebrew it's perfectly okay to omit the vav in a series of items. Indeed, he argues that in Shemoneh Esrei it would be grammatically incorrect to add it, since "Al Hanissim" is really the beginning of a new thought (which then continues in the paragraph "Ve'al kulam").
I guess most halachic authorities would recommend sticking with your community's nusach.