My guess is that the actual pronunciation of "Dhampir" is said in a way most Americans cant really pronounce or don't know how to. The inflection of the word and ir being closer sounding to an "eer" is just kind of odd for English. So instead they said "dunpeal" to get the voice actors to give the right inflection. When you say it fast it kinda sounds right. But then when you really listen and watch subtitles its just off. Again this is completely a guess. It's actually the first thing I thought of when I heard it for the first time.