PS I don’t think these are “vouchers” as such.
I have received these, but they are normally just flyers setting out how to find the Arrivals Lounge, and the services available there. They might be Concorde Room Arrivals Dining area invitations for F pax, but you certainly won’t be denied access to the Concorde Room without a “voucher” if your ticket says you are eligible. Without your ticket/Gold Card they are worthless.
T5 BA Arrivals Lounge access is available to all inbound Club World and First passengers, and all Gold Card holders. The only exception to this is the early arriving Moscow flight for those flying Club Europe, where frankly you should be given a medal, rather than just a shower.
Those travelling First have the additional Concorde Room access, and can request the infinity baths, if they enjoy an infinitesimal wait for them to fill up.
No entry is permitted without your inbound boarding pass under any circumstance. No guests are permitted, as far as I am aware, and BA does not give out guest passes either.
The situation if inbound on another oneworld carrier, or a top tier with other oneworld partners, is less clear to me, and I would imagine this is where confusion might arise.