What do you say about someone who stays up for the night vigil (qiyam al-layl) until Fajr, then sleeps through the congregational Dhuhr prayer?

A Muslim must observe prayers at their prescribed times, and it is impermissible to neglect them due to staying up late, even if it is for an act of worship. Drawing closer to Allah is primarily achieved by fulfilling obligations. Praying the entire night until Fajr goes against the Sunnah (definition) and leads to neglecting obligatory duties. The Prophet, peace and blessings be upon him, did not do that; rather, he would pray and sleep.