My Vigor 120 V2 modem suddenly requires a password, which it never had. The Draytech website tells me the default username / password is admin / blank, but it won't accept this combination. So I decided to reset the modem by switching it off / on, simultaneously depressing the reset button with a pin. You would have thought that this would reset the username/password to the default, but no. So either the modem is not actually resetting (I've tried at least 5 times) or a reset fails to reset the password. Does anyone have any ideas?

I think you are trying to reset with wrong way
It will not reset it if you press the same time the power on & reset but it may set it into TFTP mode
try

While the modem is running, press the Reset button and hold for more than 5 seconds.
When you see the ACT LED blinks rapidly, please release the button. Then, the modem
will restart with the default configuration