Do V-Bucks expire?
No. V-Bucks don't have an expiration date. Once they're in your Epic Games account, they stay there indefinitely until you use them.
Do V-Bucks disappear if unused?
No. V-Bucks are permanent as long as your Epic Games account is active. It doesn't matter how long they go unused.
What happens to V-Bucks if I uninstall Fortnite?
Nothing. V-Bucks are tied to your Epic Games account, not the game installation. If you reinstall Fortnite, your V-Bucks are still there.
Do V-Bucks expire if I switch platforms?
No, as long as you use the same Epic Games account. V-Bucks are cross-platform: if you buy them on PC and switch to PlayStation, you still have your V-Bucks.
Exception: account closure
If Epic Games closes your account for violating terms of service, you'd lose your V-Bucks along with all account content. That's why it's important not to use unofficial methods.