Shalom Daniel,

Good catch. In fact, in most manuscript versions of the Gemara, the text reads "Nidar v'Nidav," just as it appears in the Mishnah. This is already noted in the Dikdukei Sofrim, and today most manuscripts of the Bavli can easily be consulted through the "Hachi Garsinan" project.

Accordingly, there is little reason to attach significance to the reversal found in many printed editions. It is quite possible that the abbreviation "Nid' and Nid'" was inadvertently expanded in the wrong order by an early printer or typesetter. Since the manuscript evidence overwhelmingly supports the reading "Nidar v'Nidav," it does not appear that the reversal found in many printed editions is significant.

In short, your question is a good one, and the most likely answer is that the original text of the Gemara simply matched the wording of the Mishnah.

Kol Tuv,

Aharon Steiner