ok so here's how it is.
there are all types of steroids common among all sorts of champs but vayne uses one called off-damage or she spams true damage here and there.
After the math is done if you counterebuilt her specifically she should be reducing the effectiveness of your defenses about 15-25% or more if she built attack speed.
If you look here you can see how much she's doing this on average
http://www.leagueofgraphs.com/champions/stats/vayne
in gold+ it tends to be about 20% true damage but then that's not quite a measure of how much she ignores per say since you don't really know how much physical was reduced. Anyways, vayne is going to shred your defenses. That's just what she does and there's really nothing wrong with it. If you build inefficient defense items then in some cases you may as well not be building anything or more accurately you may as well be getting 20% less or 25% if you aren't getting a lot of armor to go with an appropriate amount of hp (as in not too much or you are getting less for your buck)
That aside, vayne also has an aa reset on her Q and a damage amp as well but then she also gets AD from her ult. Under the right conditions with attack speed she will destroy you regardless of what you built if you can't kill or escape. This is something adcs are designed to do really. Vayne just has an intense to watch looking version.