Is someone who prays on Friday night only—and in Ramadan, performs vigil on some other nights—rewarded for that, or is it considered an innovation? And does refraining from praying on some Friday nights not become an innovation?

It is disliked to single out the night of Friday for night prayers (Qiyam), because the Prophet, peace and blessings be upon him, forbade that. However, if one performs night prayers on the night of Friday sometimes and leaves it sometimes, there is no harm in that, provided it happens incidentally, depending on one's free time and the availability of means for Qiyam on that night, and not as a stratagem to intentionally single it out.