Sorry, I don't know. Probably expires in 30 days, if a quick call isn't made? You should ask Vodafone.
When I had my Nokia flip-phone years ago it was before Internet and texts. So my top up vouchers were for minutes/calls only. I rarely used my mobile, and when I did, it was for quick calls lasting no more than 2 minutes. That's how my top up(s) were still going two months later.
Nowadays top up vouchers are eaten alive by Internet data usage, etc. But the link I provided tells you how to turn that off in the FAQ section. Depends on if you just want to make and receive calls with your emergency phone (and not use the Internet bit).