Is praying daily as fulfillment of a vow an innovation (bid‘ah)? Is it permissible to vow to recite the Quran? Is expiation (kaffarah) obligatory for not fulfilling it?

Your vow to recite two Surahs before sleep every night is a vow of obedience, and you are obligated to fulfill it, based on Allah’s Almighty saying: "And let them fulfill their vows," and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him."

There is no innovation or detestability in this; rather, what is disliked is making a vow contingent upon a condition.

You are not required to offer expiation unless you become unable to fulfill the vow, in which case an expiation for breaking an oath becomes obligatory.

Your prayers with the intention of fulfilling the vow are valid and necessary, because "deeds are by intentions."