I took a vow to pray in congregation under any circumstances, but sometimes I fail to do so because of work conditions. I may delay the prayers due to some circumstances that are beyond my control but perform them in congregation, if only with one of my co-workers. Are those prayers considered congregational prayers?
Thank you.

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.

If by saying, "to pray in congregation," you mean performing the congregational prayers in the masjid, then your performance of the congregational prayers in a place other than the masjid is not considered a fulfillment of your vow. When a person takes a vow to perform the obligatory prayers in congregation, he is vowing to do an act that is already deemed obligatory by the Sharee'ah. According to the preponderant scholarly opinion that we adopt, it is obligatory for Muslim men to observe the congregational prayers in the masjid. When the person takes such a vow, he is vowing to do an act that is already obligatory by the Sharee'ah. Such a vow is not valid or binding, according to the opinion of the majority of scholars.

However, you added the phrase, "under any given circumstances," in your vow, and this renders your vow a vow to do an act that is not already obligatory by the Sharee'ah. This is because you are not obliged to perform the congregational prayers in the masjid in the case of inability. Thus, you vowed to do something that is beyond your ability and is not obligatory in Islam. When the person takes a vow to do an act beyond his ability, he should offer an expiation like that for breaking an oath. Ibn ‘Abbaas narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever makes a vow that he is not able to fulfill, the expiation for that vow is like the expiation for breaking an oath." Al-Hafith Ibn Hajar said, "The isnad (i.e. chain of narrators) of the hadith is sound, but the hadith scholars and critics are more of the opinion that it is a Mawqoof hadith."

However, if you intended, by your vow, to perform prayers in congregation in general, without specifying performing them in the mosque, then the least number of persons required for the validity of the congregation is two. If you performed the prayers in congregation with another adult man, then you fulfilled your vow. The following is stated in Al-Mawsoo‘ah Al-Fiqhiyyah:

"Islamic jurists agreed that the least number of people required for the validity of the congregational prayer is two: the Imaam (i.e. one leading the prayer) and another one with him. This ruling is not applicable to the Friday and the 'Eed prayers. The majority of Islamic jurists stipulated that it is required for the validity of the congregational obligatory prayer that both the Imaam and the one led in prayer should be adult Muslims, even if the one led in prayer is a woman. According to their view, the congregational obligatory prayer is not valid if the one led in prayer is a young boy because the prayer is obligatory for adults and voluntary for the boy. As for performing voluntary prayer in congregation, according to the consensus of scholars, it can be established by two boys or an adult imam and a boy. The apparent meaning of the opinion of the Shaafi‘i scholars, which is also one of the opinions reported from Imaam Ahmad is that the congregational obligatory prayer is valid with an adult Imam and a young boy who is led in the prayer as well." [Al-Mawsoo‘ah Al-Fiqhiyyah]

Allaah Knows best.