Sorry to hear about your dad, this must be tough enough for you without trying to deal with Vanquis.
If they pass it to s debt collector, simply write and tell them the debt is in dispute and they must refer it back to Vanquish they then have to do this by law. Disputed debts cannot be chased by DC's.
I would take this to the FOS as if nothing else, it means they will take the matter off your hands for a while.
Not sure what the outcome will be tbh, the debt still exists but think I would go for the mis-selling angle, it's irresponsible lending if nothing else.
Once you have made your complaint to the FOS, just copy and forward any further correspondence straight to them.
You concentrate on your dad and if you need any further help, please give us a shout. More than happy to help you with letters etc.