A so-called Vulture Fund is an investment fund that will seek out and purchase securities in distressed investments. The objective thereafter for the Fund is to 'swoop in' and pick up under-priced assets at a fraction of the debt, with a view to collecting from debtors an increased percentage of the bad debts. This is particularly true within the European Banking sector in relation to non-performing property mortgages/ loans.
For more Information - https://www.judicaregroup.com/site/country/lawyers-in-spain/mortgage-problems-in-spain/vulture-fund/
These funds then aggressively pursue the mortgage defaulters seeking to recover the amounts.  Companies like Cerberus, Blackstone and Axactor to name a few of the entities chasing clients in the UK.