There are 2 purging wands in act one plus the helm. That is enough for 12 and there are only 6 shriekers. You don't actually need to destroy any of the shriekers anyway as you can either
befriend Slane the dragon who will kill them
or
Cloke and Dagger past them
You don't need a purging wand in act 2 at all
You will learn source vampirism as part of the main quest and the range is always sufficient
I think you do need them on Nameless Isle but there are hundreds lying around and you can skip that bit anyway if you want.
There are some people you possibly shouldn't kill in Act One (Sahelia for example) but they only block side quests later on.
I don't believe you can scupper your main quest progress by killing the wrong people - just lose some incidental stuff.
