To be clear, are you referring to the whole "fan-based project" spiel? If so, one way of going about this would be to declare a content addition with :after.
In this specific instance, we can (somewhat hackily) declare that addition on top of the italics tag, since the disclaimer is the only piece of text in that footer that's using italics;
<style>
footer p i:after {
display: block;
content: "Copyright © Vault-Tec, 2077";
}
</style>
It's not the prettiest solution - chiefly because content additions don't appear in the DOM (i.e. they don't count as text that's actually on the page, if that makes sense) and, as such, are invisible to screen reader programs and whatnot - but given that we're limited on options here, it'll suffice.