Vector images, store image information in the form of mathematical formulae. And since a mathematical formula doesn't have a "resolution", we are allowed to rasterize a vector image at whatever resolution we want.
Converting vector images to bitmaps is possible, but we can't go the other way around without losing information. It would be impossible to convert a bitmap into a true vector. We can approximate a vector representation of a bitmap, but we can't truly convert from raster graphics to vector.