She's obviously used the vents to escape the Aliens at some point in the past. But she never stayed inside them all the time. she was roaming about the complex when the Marines arrived, so I bet she's done that several times in the past risking her own life. Like, you only have to see the amount of resources she has in her makeshift home to see that's she's been hording. So I bet she was still taking a chance when leaving the vents to collect vital resources. Obviously by the time the Marines arrived the xeno activity was low since most/all were hibernating. But still, you can't say it was pure skill and intelligence why she survived for so long. It's obviously a mixture of a lot of things, like her being good at hiding in the vents and also pure luck that a xeno ain't clocked her.