I Don't Understand How Vi Works

So ARAM is not the Rift, so a lot of things don't really apply there. However my last two matches with Vi on ARAM have been absolutely abysmal. I just don't seem to understand how she's supposed to function. I can ult on someone, following up with Q and E, trigger W, and then ... that's it. I've done maybe a third of their health and they turn around and kill me. Like I said, these are ARAM experiences, so it isn't really a big deal. But if someone could give me a brief rundown on Vi tactics (or direct me to someone who could) so that next time it's not so bad I'd really appreciate it.
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