there have been a few posts in the past about these accomodation vouchers. from what I understand , you are required by Indian law to have somewhere booked for the first night or you are not allowed to board the plane in the UK.
Also on the swine flu forms you need to put an address in Goa
So when you book flight only , one nights accomodation is always included so as to comply with the law.
Not sure if this applies to sheduled flights or just to charter flights.
I have no idea if the voucher is actually valid when you get there, or what kind of accomodation it would be.
If the Park Avenue Annexe is part of the hotel of the same name it sounds pretty basic
From what I have read on here in the past, people seem to accept the voucher and then forget about it when leaving Dabolim.
If you are worried though, contact your agent.