Interfaces can still be expressed using vtables. You just have to write the vtable yourself rather than have the language do it for you.

Also, Zig's tagged unions (enums with payloads in Rust) are really ergonomic and often what you want instead of interfaces. Alot of languages that use interfaces simply don't expose a good way of doing it so everyone reaches for interfaces by default. But if you don't need an actual interface then this way you don't even have to pay the cost of runtime dynamic dispatch.