it literally takes no timing. you just mash it. Just have the computer do vegetas assist or him stand alone or whatever. Works with gotenks as well. It should be impossible almost to not get it by mashing.
So basically anyway just saying the japanese saw a lot of vegeta assist and then people doing block string but fact is again I showed it in my video. last hit counts as a fireball so you can then cancel strings easily. Its only somewhat better.