There are vegan blood meal alternatives out there to resolve this exact conundrum.

But the reality is, unless your plants are being grown hydroponically in a sealed warehouse or similar, chances are real good that they are feeding on decaying animals (either directly or indirectly) whether you like it or not. They're mostly insects and annelids and such, but still animals.

I think the issue for vegans is more about whether animal slaughter was involved in making their fertilizer. Dead pillbugs in the soil is just nature doing its cycle of life thing.