What is the ruling on vowing to pray two supererogatory rak'ahs after 'Isha daily, with Surat Al-Fatih and Al-Ikhlas, as an expression of gratitude to Allah, and on reciting Al-Fatihah and Al-Ikhlas in the second rak'ah of every obligatory, Sunnah, or supererogatory prayer?

You are permitted to pray two rak'ahs after 'Isha with Surat Al-Ikhlas, based on what is authentically reported in the Hadith, that the Prophet, peace and blessings be upon him, said about the Companion who concluded his prayer with Surat Al-Ikhlas: "Inform him that Allah loves him." And based on the Prophet's, peace and blessings be upon him, saying: "Whoever vows to obey Allah, let him obey Him." You are obligated to recite the specific surah as long as you have vowed it. As for your vow to recite Surat Al-Ikhlas in the second rak'ah of every prayer, there is a difference of opinion regarding it, and the preponderant view is that you are not obligated to fulfill it, because it is a change to the Sunnah.

And Allah knows best.