The source in the Machzor Vitri may be found in Pirkei Avos, perek 3, mishnah 14. In the Nirenberg edition of Shimon Halevi (Ish) Horowitz , 1923, it can be found on page 514.
It should be remembered that in those times, fierce polemics existed between the Jews and Christians, and often still with the Karaites. Much of what was written was affected by those theological battles. Therefore, it is a mistake to draw conclusions as to apikorsus generally, and who is an apikores individually. Be careful what accusations are made,the apikorsus sword is double edged.