Why do Chabad siddurim always print Veshameru before the Shabbat Ma'ariv Amida, even while also noting that the Chabad custom is not to say it? Are there any groups that use the Chabad Nusach Ari siddur, and include Veshameru?
Two stories explaining why the Alter Rebbe included V'shomru in his siddur:
R' Avraham Chaim Na'ah, in his sefer Piskei Hasidur (paragraph 128), brings a story Chassidim would tell.
Levi Yitzchok of Berditchev once asked the Alter Rebbe why his custom is not to say V'shomru, if it makes such a "יריד" (usually translated as fair, or parade) in heaven. The Alter Rebbe answered that juxtaposing Redemption to the Amidah also makes a fair in heaven, and you can't be at every fair.
But out of respect for R' Levi Yitzchok of Berditchev, the Alter Rebbe included it in the siddur, with a preface saying our custom is not to say it.
Rabbi Abraham J. Twerski, in his book "The Zeide Reb Motele", tells a different story that was a tradition in his family. He says when his ancestor, the grandson of R' Menachem Nochum of Chernobyl (Yaakov Yisroel of Hornsteipl) married the granddaughter of the Alter Rebbe, The Alter Rebbe asked R' Yaakov Yisroel to start saying "Ki Vanu Vachartah" in Kiddush on Friday night. R' Yaakov Yisroel answered that he would if the Alter Rebbe would start saying V'shomru. The Alter Rebbe answered that he was too old to change his ways, but as a compromise he would include it in the Siddur. (see here)
--- Dayan Raskin quotes a Gilyon HaBeSH"T that wants to disprove this story on the grounds that the dates don't match up. (If someone could provide a link to that Gilyon HaBesh"T that would be great)