I am supposed to fast on Monday as a vow, but in the same time I have been fasting for expiation, and I fasted Monday, but I am very confused, whether this fasting will be for the vow or expiation. Before I fasted that day, I expressed my Niyah for both. Is this correct since I am still in fasting for expiation?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Intending to perform two good deeds by one act is possible in some acts like Hajj and Umrah or Ghusl and Wudu.  But it is not possible in other acts including fasting.  So, you cannot have two intentions for fasting one day.

From the above you know that you have to redo the expiation fasting of that day and make up for the day of vow (Nadhr).

Allah knows best.