Actually Vector images can become much more expensive to render than Bitmap images because Vectors images use mathematical formulas to determine size and shape. Since a Bitmap does not do this, you only incur the cost of storing the collection of texels in the image at it’s current resolution. Also Bitmapped images also make use of Mip maps which are like LOD’s for your textures.
This is the reason that you do not see Vector images used a lot in video games because they could quickly eat up any available resources and there is no Mip Mapping. I do know that they do you can use Signed Distance fields for font rendering and the the UDK did this so I assume that UE4 does this as well. Signed Distance fields is a fancy way of giving you much higher quality rendering on things like fonts and shadows. You can read more about them from this Value paper.