I made a vow to Allah, the Exalted, that I would perform two rak‘ahs (units of prayer) before every obligatory prayer throughout my entire life as an expression of my thankfulness to Allah, the Exalted. However, one day, in Ramadan, as I was performing prayer in congregation with my parents, they rushed me to begin the prayer; so I could not perform the two rak‘ahs before it, but I performed them after the obligatory prayer. What is the ruling on that?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Performing the two Rak‘ahs after the obligatory prayer was the right thing to do.  ‘Aa’ishah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to do an act of obedience to Allaah, the Exalted, must do it." [Al-Bukhari]

Moreover, you are required to offer an expiation like that of breaking an oath as you failed to fulfill your vow in its due time.

We would like to point out two issues for the questioner. Firstly, if the (fulfillment of the) vow becomes due at a time when it is disliked to pray, then you are not obliged to fulfill it. Secondly, performing prayer in congregation is not obligatory on you (the questioner is a female). Therefore, you should give precedence to fulfilling your vow over joining the congregational prayer in the future.

Allaah Knows best.