Oddly enough, vim has had this functionality for ages (at least since the mid-90s; ^K triggers it as the "compose" key), but they broke it in a fundamental way during an "upgrade" a few years back. Originally, the compositions were as described by Mr. Sivers: just the pair of keys you'd expect, for instance a + ^ to get â. But then, for no apparent reason, they changed all the defaults to totally unobvious things: it's now a + > to get â, and a + ! to get à, and a + ? to get ã. You can reprogram your own, but the whole point of the system was that you'd just know, which is no longer true, alas.