Before us is Mishnah 10 in chapter 3 of Maseches Nedarim, which deals with a person who vows not to derive benefit from those who keep Shabbos. The central question the Mishnah addresses is who is included in such a vow: only full-fledged Jews, or the Kusim as well.
The words of the Mishnah:
"HaNoder mi'shovsei Shabbos - assur b'Yisrael v'assur b'Kusim; me'ochlei shum - assur b'Yisrael v'assur b'Kusim; me'olei Yerushalayim - assur b'Yisrael u'muttar b'Kusim" - one who vows not to benefit from those who rest on Shabbos is forbidden to benefit from Jews and forbidden to benefit from Kusim; from those who eat garlic, he is forbidden with Jews and forbidden with Kusim; from those who go up to Jerusalem, he is forbidden with Jews but permitted with Kusim.
Background: who were the Kusim?
The Kusim were groups of people brought by the king of Assyria to replace the ten tribes. The Gemara records a dispute as to whether their conversion to Judaism was valid or not. At a certain point, in the time of the Gemara, they were found to be worshippers of idolatry, and from then on they were treated as full-fledged gentiles. In the time of the Mishnah, however, there are opinions that considered them valid Jews.
The wording of the vow is what determines:
Whether the one who vowed intended to include the Kusim in his vow depends on the type of expression he used. The three wordings in the Mishnah teach us this:
"Mi'shovsei Shabbos" - from those who rest on Shabbos. The Kusim also kept Shabbos. They observed everything explicitly stated in the Written Torah, and their problem was only with the Oral Torah. Therefore, one who says he is vowing away from anyone who keeps Shabbos includes the Kusim in his vow as well.
"Me'ochlei shum" - from those who eat garlic. This was an enactment of Ezra, to eat garlic on Friday night, since it aids a person's fertility. And because Friday night was the time when marital relations were customarily observed, this served as an aid to fertility. Apparently the Kusim kept this enactment as well, even though it is rabbinic, and therefore the Mishnah rules: "assur b'Yisrael v'assur b'Kusim" - he is forbidden with Jews and forbidden with Kusim.
"Me'olei Yerushalayim" - from those who go up to Jerusalem. Here the law is different: "assur b'Yisrael u'muttar b'Kusim" - forbidden with Jews and permitted with Kusim. The one who vowed is forbidden to benefit from Jews, who are the ones who go up to Jerusalem for the festivals, but he is permitted to benefit from Kusim, since they took no part in the pilgrimage to Jerusalem. The Kusim maintained their own worship on Mount Gerizim and did not come to the Beis HaMikdash, and therefore they are not included in this vow.
In summary: the Mishnah teaches that the scope of a vow is determined by its wording. In a vow away from "shovsei Shabbos" and from "ochlei shum," the Kusim are included as well, because they kept these practices; but in a vow away from "olei Yerushalayim" they are not included, since they did not go up to Jerusalem but worshipped on Mount Gerizim.