How can I conditionally provide a default reference without performing unnecessary computation when it isn't used?

You don't have to create the default vector if you don't use it. You just have to ensure the declaration is done outside the if block.

fn accept(input: &Vec<String>) {
    let def;
    let vec = if input.is_empty() {
        def = vec!["empty".to_string()];
        &def
    } else {
        input
    };
    // ... do something with `vec`
}

Note that you don't have to build a new default vector every time you receive an empty one. You can create it the first time this happens using lazy_static or once_cell:

#[macro_use]
extern crate lazy_static;

fn accept(input: &[String]) {
    let vec = if input.is_empty() {
        lazy_static! {
            static ref DEFAULT: Vec<String> = vec!["empty".to_string()];
        }
        &DEFAULT
    } else {
        input
    };
    // use vec
}