Jesus' prayer in Gethsemane presents readers with a challenge, but not for the reason suggested in the OP. This was no "vain repetition" like chanting a mantra; it was a heartfelt plea of a son to a father. The challenge, of course, is that Jesus would not have prayed such a thing if knew the Cross was his destiny and was always willing to accept the Father's will.
Regarding Luke 6:12, a prayer that continues all night DOES not imply a "vain repetition" or "many words" in the sense meant here. All night prayer vigils can take various forms. Repeating a liturgical formula over and over is only one of them. Babbling on endlessly might be another. But an all night prayer vigil might also represent long, tearful pleadings to see the "face of God" (Ps. 42) or intimate discussions in which the petitioner seeks God guidance. This seems to have been the case in the OP's example, as well as in Luke 9:28 where Jesus consults with the spirits of Moses and Elijah. It could also take the form of sitting in silence and communing with God.
Conclusions: The supposed contradiction is explained in the case of Matthew 26:44 by the fact that Jesus did not merely repeat a mantra or liturgical formula but engaged in a heartfelt pleading with God. It is explained in the case of Luke 6:12 by the fact long prayer vigils may involve prayerful petitions and conversation with God was well as "much speaking" in the pejorative sense.