Thanks for the question.
You certainly can adapt your vow, which is really not a vow in the strict sense.
A promise of this nature is more like a promise to oneself. God will not hold you to it as He would a man to his wife in marriage. He looks at it like an earnest commitment to spend time with Him, to make a routine sacrifice and to stick with the Christian life. You are in charge of the specifics. So, if you can't do the first promise, just acknowledge it to yourself and God by saying,
"I'd like to offer up this creed today instead of reading a chapter as I originally hoped."
He'll honor that as if it were the original intent. You can make changes as you go along, but the point is to stay committed to prayer, to study, and growth in faith, hope and charity.
An important vow is to do something everyday, perhaps a set of a prescribed prayers like the Rosary, the morning offering and examination of conscience before bed. The habit should be something you can maintain without too much variation.
For example, you can pray the Rosary in parts, a decade here or there, on the way to class, on a study break, etc. Do the morning offering when you wake up, and your final prayers when you are tucking into bed. Sleeping and rising are things you do everyday so if you tie something routine to that it will become easier to maintain.