במדבר פרק ל, ב וַיְדַבֵּר משֶׁה אֶׁל-רָאשֵּי הַמַטּוֹת לִבְנֵּי יִשְרָאֵּל
The speaking directly to the heads of the tribes is unique. It is possible to explain that even though the Gemara (Menachos 73b) establishes that a non-Jew may make vows (נדרים ונדבות), however, the concept of release of vows by a Chacham is only applicable to the Jews. Thus, the verse’s emphasis of giving the commandment to the heads of the tribes about the Jews, limits the concept of release of vows only to the Jews.
במדבר פרק ל, ג לֹא יַחֵּל דְ בָרוֹ כְכָל -הַיֹצֵּא מִפִיו יַעֲשֶׁה:
Apparently, after the verse states that one should not “profane” one’s word, it is redundant to state that one should do according to what comes out of one’s mouth.
It is possible to explain that the additional language alludes to what is taught by Rambam (Hilchos Deios - Laws of Personal Development, first chapter) that in all matters, a person does not go to extremes, but should take a middle course. This is supported by the words of Shlomo HaMelech (קהלת ז, טז) אל תהי צדיק הרבה ואל תתחכם יותר למה תשומם. Further, we find the concept that Tzadikim say little and do a lot, and conversely, Reshaim say a lot and do very little.
Thus, the verse is providing the additional concept that one should not turn to either extreme. One should not add excessively beyond one’s vows, nor should one refrain from them. Rather the proper course is to do exactly as one states, neither less or more.
במדבר פרק לא, ב נְקֹם נִקְמַת בְנֵּי יִשְרָאֵּל מֵּאֵּת הַמִדְ יָנִים אַחַר תֵּאָסֵּף אֶׁל-עַמֶׁיךָ :
This verse calls it the vengeance of the Jews, but in the next verse, Moshe tells the Jews that this is the vengeance of HaShem. Why does Moshe change the words of HaShem?
The simple explanation is that HaShem honors the Jews by using the term vengeance of the Jews. Responsively Moshe honors HaShem by using the term vengeance of HaShem. (Similarly, in Parshas Shlach, Moshe calls the place מסה ומריבה, using the term מסה first to honor HaShem; whereas HaShem refers first to מריבה in order to honor Moshe, as explained there.)
It is also possible to explain that HaShem said immediately afterwards that Moshe would be gathered to his people. Thus, the fulfillment of the vengeance would lead to the passing of Moshe. Thus, the people approached Moshe and asked if they should delay in taking the vengeance in order to prolong Moshe’s life. Thus, Moshe responded that this matter is also vengeance for HaShem, and they should not delay carrying it out.
Why did HaShem connect the taking of vengeance against the Midyanites with the death of Moshe?
Perhaps, one could explain according to the Gemara (Moed Katan 28b) that the verse (Yermiyahu 34,8) that Tzidkiyahu passed in peace. This verse alludes to the fact the Nebacudnezer died first. Thus, Tzidkiyahu was able to die peacefully knowing that his evil rival was dead and no longer a threat to his nation. Similarly, Moshe would be able to die peacefully only after knowing that the vengeance was completed against the Midyanites (and Moabites) for the evil actions against the Jews.
