If you look at the String documentation, there are a few methods you could use. There’s String::from_utf8 that takes a Vec<u8>, and there’s also String::from_utf8_lossy which takes a &[u8].
Note that a Vec<T> is more-or-less an owned, resizable wrapper around a [T]. That is, if you have a Vec<u8>, you can turn it into a &[u8], most easily by re-borrowing it (i.e. &*some_vec). You can also call any methods defined on &[T] directly on a Vec<T> (in general, this is true of things that implement the Deref trait).