@TransTech Vector graphics uses mathematical formulas to represent the image. This allows resizing the image without degrading the quality.
More details here: http://en.wikipedia.org/wiki/Vector_graphics

The code, ui and graphics done for the N900 will be also used on the N9 and possibly other devices and they have various screen sizes.

If you embed a bitmap into inkscape then you haven't converted it to vector. You just embedded it.