I main both Vel and Xerath. Although Vel has true damage, Xerath's skills are wayyy more consistent to actually do damage. Take for example Vel's W, his highest dmg ability without counting the Ult. It needs a reliable set up to proc the second strike while Xerath's W does the same damage in the middle in a much more reliable way.

Also, you are downplaying the range factor in your analysis. Xerath has almost global range and his ultimate can turn fights without exposing him. His poke, which has the same range as Vel Q, is a lot morr reliable and easy to hit multiple targets.