If the vendor has obtained home loan from the bank then without discharging the loan amount the bank will not release the original title documents held in its deposit.
The vendor will be having the copy of the documents deposited by him with the bank for availing the home loan.
In that you can find that if the original title documents are held by bank, then obtain a certified copy from the registrar's office and forward the same to your lawyer for obtaining the legal opinion.
It is the duty of the vendor to provide the original title document, he cannot claim excuse that the bank has lost it hence he cannot produce the same.
The registrar will not register the sale deed on your name if the original title document is not presented before him.
If the bank is reporting loss of the original title document held in their deposit then they have comply with the procedures as per law to provide the prescribed certificate to the registrar in order get the property registered by furnishing the certified copy in the absence of the original document as per due process of law for loss of original documents.