My Vodaphone Pay as you go requires that you make or receive a call at least once every six months. This seems to include texts as well. Just call your home phone now and then to keep it working.
My old mobile is about 8/9 years old and like you I rarely use it and it is switched off most of the time. I just switch it on about once a week and charge it when necessary.
I think you can call Vodaphone and get the old Sim reactivated.
I too have a T Mobile Sim card (Freebie) but it won't work in my Vodaphone hand set, probably because the Vodaphone is locked to their Sim.