The gemara (Nazir 3b) quotes a derasha to prohibit a nazir from mitzvah wine as well as non-mitzvah wine. What mitzvah wine are we talking about, asks the gemara? The gemara answers: kiddush and havdalah. But, asks the gemara, why is a derasha needed to permit that – the nazir is “mushba v’omeid” already? Rashi explains: since the Torah commands that kiddush be made over wine, no vow of nezirus can interfere with that pre-existing obligation.
Tosfos disagrees with Rashi's reading. Reciting kiddush over wine may be a din d'oraysa, but drinking the wine of kiddush is certainly is not! The gemara, explains Tos, is asking a rhetorical question – is the person mushba v’omeid to drink the wine, requiring a pasuk to teach that it is allowed?! Obviously not, therefore the pasuk must teach us something else.
Did Rashi really think that even drinking wine is part of the mitzvah d’oraysa of kiddush? Possibly, but it is difficult to understand why this should be so. Rav Soloveitchik suggested a different approach to Rashi (quoted by R’ H. Schachter in “M’Pninei haRav”, Zeved Tov p. 249). Rashi read the gemara to mean that the cheftza of mitzvah wine is mufka, inherently excluded, from the parsha of nezirus. In other words, wine used for kiddush is a food-item which is excluded from the nezirus vow, just like most other food and drink. It's not that the mitzvah of kiddush overrides the vow of nezirus, which would force us to define kiddush as including drinking, but rather that kiddush wine is never included in that vow in the first place.