The Vampyre gets angry. When angry, its appearance changes to that of a Feral vampyre, and can be hurt by any weapon (very common).
If a Guthix balance potion is not used, then the Juvinate will say "Ha ha, time to disappear and re-juvinate!", turn into a cloud of mist and float away when it reaches approximately 0 health. No drops will be received when this happens. The ones in Darkmeyer are immune to the balance potion and do not turn into mist. They also count for VampyreSlayer tasks.
Vampyre Juvinates and Vampyre juvinates used to be significantly different, the latter having a much older, albeit unique, model, despite being released later. The graphical updates to Morytania and vampyres solved this, making the two look the same. The combat levels and capitalisation remained different though.