I was chanting almost for an year, but my mind still keeps away from getting focussed to it. Why does it happen and what is the purpose of chanting if this continues?
Chanting is a way of declaring war with Maya with the support of Sri Krishna and the effect of war is making us diverting the attention. Because of Lord's support, chatting would certainly win if the chanting continues.

When we're are part and parcel of the Lord, where does Dwaitham or Vishistadwaitham exist?
There are three types of energies of the Lord. Out of that the marginal energy is the lower most and all souls are from that energy. Hence, Lord stays in the heart of all soul to guide.

What am I? Am I the soul only or the subtle body and the soul? Please enlighten me.
You are the soul, but you are presently covered by two types of bodies: the gross body and the subtle body. The gross body is composed of earth, water, fire, air, and ether. And the subtle body is composed of mind, intelligence, and false ego. Beneath these two coverings is the actual self, the soul. The gross body is given up at the time of death when you accept another gross material body according to the state of your consciousness at the time of death. The subtle body goes with you birth after birth until you become completely free from its clutches and go back to the spiritual sky for reviving your original spiritual form. The gross body is given up at the end of each lifetime in exchange for a new gross body either as a human being, a plant, or an animal. And the subtle body travels with you through your millions of births until it is finally given up at the time of your spiritual emancipation.
Sankarshan Das Adhikari