I don't think it's a great idea to use vibe coding on an existing project like GIMP unless you want to fully convert it to vibecoding. The moment you start vibe coding major features you lose all oversight of what the code does.
That's why I think this is a great way to use vibe coding. Just clone the entire Photoshop project and re-implement it all with vibe coding in Rust.
Rust is a difficult language to write compared to something like JS, but at the same time it's memory safe and can prevent a lot of bugs. That's why it's a great language for AI to use.