A few quotes from the Rabbi on 'Customs of Israel'
in which he allows eating meat in an obligatory festive meal [se'udat mitzvah] despite the custom to refrain from eating meat from the 17th of Tammuz
The custom to refrain from eating meat from the 17th of Tammuz is the equivalent of a vow. However, if one has the opportunity to participate in an obligatory festive meal they are permitted to eat meat, for it is not actually a vow but is considered as having accepted to undertake a fast. It is like saying "I will fulfill this commandment" when it is obligatory by law. Any case of a mitzvah is permitted, since a vow to fulfill a mitzvah is not applied at the expense of another mitzvah.
Machaneh Ephraim, Halakhot on Vows, Section 40, p. 77, Or Haim – New Press, The Center for Jewish Books, Jerusalem 2011
