Just to review--there are actually thirteen tribes when we include the tribe of Levi. We ended up with an 'extra' tribe when Yaakov effectively makes Yosef into two tribes by giving each of his sons (Efraim and Menashe) a portion in his inheritance.
In any event, all of the n'siim bring their dedication sacrifice except for Aharon who was the nasi of the tribe of Levi. Rashi brings a midrash here at the beginning of the parsha which indicates that Aharon was crestfallen to not be included in the lineup of his fellow n'siim but God comforts him by telling him that he has the mitzva of lighting the menorah which is even greater than what the other n'siim did. The Ramban expounds on that midrash and tells us that it is an allusion to a time in the future when the Hashmonaim, who were priests (descendants of Aharon from the tribe of Levi, of course) will rededicate the Temple through the miracle of lighting the menorah for eight days, as we remember on Hanukah.
However, it occurs to me that something else may also be at play here. Just after the first verses in the parsha deal with the lighting of the menorah, Moshe is commanded to prepare the Levites for their service in the tabernacle.
The process for the preparation was to entirely shave the bodies of the Levites, to prepare a sacrifice to bring with them and then it says:
במדבר פרק ח (ט) וְהִקְרַבְתָּ אֶת הַלְוִיִּם לִפְנֵי אֹהֶל מוֹעֵד וְהִקְהַלְתָּ אֶת כָּל עֲדַת בְּנֵי יִשְׂרָאֵל:
(י) וְהִקְרַבְתָּ אֶת הַלְוִיִּם לִפְנֵי יְקֹוָק וְסָמְכוּ בְנֵי יִשְׂרָאֵל אֶת יְדֵיהֶם עַל הַלְוִיִּם:
(יא) וְהֵנִיף אַהֲרֹן אֶת הַלְוִיִּם תְּנוּפָה לִפְנֵי יְקֹוָק מֵאֵת בְּנֵי יִשְׂרָאֵל וְהָיוּ לַעֲבֹד אֶת עֲבֹדַת יְקֹוָק:
(י) וְהִקְרַבְתָּ אֶת הַלְוִיִּם לִפְנֵי יְקֹוָק וְסָמְכוּ בְנֵי יִשְׂרָאֵל אֶת יְדֵיהֶם עַל הַלְוִיִּם:
(יא) וְהֵנִיף אַהֲרֹן אֶת הַלְוִיִּם תְּנוּפָה לִפְנֵי יְקֹוָק מֵאֵת בְּנֵי יִשְׂרָאֵל וְהָיוּ לַעֲבֹד אֶת עֲבֹדַת יְקֹוָק:
Bamidbar Chapter 8 (9) And you shall bring forth the Levites before the Tent of Meeting (a.ka. the Tabernacle or the Mishkan) and you shall gather together all of the congregation of the children of Israel. (10) And you shall bring close the Levites before God and the children of Israel will lean their hands upon the Levites (11) and Aharon will wave the Levites (in a) waving before God from the children of Israel and they will be to do the service of God.
What is happening here is really nothing less than the Levites being offered up as a kind of live sacrifice to God; i.e. the process they are going through (having other lean their hands upon them, being waved not to mention the fact that the verse says literally they are 'being offered').
So in fact it would seem the main reason that the Levites do not offer a sacrifice of dedication to the Tabernacle is that they themselves are the sacrifice-their service in the Temple is an ongoing sacrifice/dedication to the Tabernacle and to God.
The interlude about the menorah, then could be understood more simply than the Ramban's expansion on the midrash. It just means that the lighting of the menorah is a unique and integral part of the service which is the Levites' offering.
Why the emphasis on the menorah? Perhaps more than any other aspect of the service, the lighting of the menorah illustrates the notion of the light of the Torah being brought into this world but I will have to leave that for another posting.
Shabbat shalom!