I was wondering, if a person makes a vow to renounce an act that is recommended, such as not reading/researching hadiths for a period of time or voluntary prayer, is it considered valid?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ʻalayhi wa sallam, is His slave and Messenger.

When a person swears by Allah to neglect a Sunnah prayer or to refrain from reading hadiths, his oath is binding and it is recommended for him to break his vow and pay the Kaffaarah (expiation) of breaking the vow. Ibn Qudaamah said, “If the oath is to do a disliked deed or to neglect a recommended one, it is recommended to break it. The Prophet, sallallahu ʻalayhi wa sallam, said, ‘When you take an oath and consider something else to be better than it, make expiation for your oath and choose the better alternative.’ He also said, ‘By Allah, Allah willing, if I ever take an oath (to do something) and later on I find something else better than the first, then I do the better one and give expiation for the dissolution of my oath.’” [Al-Mughni]

The expiation for breaking an oath is to feed ten poor persons or to clothe them or to free a slave. If one cannot afford any of these three options, he should fast three days.

Allah knows best.