Board Thread:General Discussion/@comment-74.138.76.27-20130303123606/@comment-1738746-20150926093105

It could be argued that the vampire has either ingested some kind of special potion or using some kind of magic to disguise these effects in order to blend in with humans better, considering that's what it's aiming to do. However, I have heard of other vampire NPCs showing up under the Detect Life spell as opposed to Detect Dead, so it's highly likely to be an oversight on Bethesda's part :P