Is what the questioner committed himself to, in terms of prayer and charity, considered a vow, and is it permissible for him to refrain from fulfilling it despite being able to do so?

A vow (nadhr) is not established except by an utterance that indicates commitment, such as "It is for Allah upon me such and such" or "I vowed such and such." If you utter a phrase that signifies commitment, then this is a vow, and you must fulfill it. It is not permissible to abandon it. Rather, you must pray the specified rak'ahs and give the mentioned money in charity. However, if the commitment was merely by intention or by a phrase that does not signify commitment, then this is not a vow, and you have the choice between continuing it or abandoning it. There is no sin upon you for abandoning it, nor is there any expiation. It is better to continue the good deed that you accustomed yourself to, for the most beloved deeds to Allah are those that are continuous, even if small.