You are right, there are some cases where vector graphics are not as good as traditional bitmap graphics (especially when the vector graphics weren't build for the size they are being rendered at).

But as soon as you don't know the pixel density of the target screen, vector graphics produce much better results compared to the time you would have to invest in perfect bitmap graphics.

So if you are designing a website (potentially viewed on devices with different pixel densities), vector graphics are likely to outperform bitmap graphics.