Traditionally, very dark rituals were done to connect with death such as drinking blood or consuming the flesh of dead bodies. That isn’t vegan if you consider human corpses to be animal products and can even get you into legal trouble. However in the modern times, there are some fun, easy recipes that can be used in place of human flesh and bones.
The first and most obvious alternative is to use Tofu or soy products. You can even buy the beans and go through the process of turning them into milk, tofu, or beans which will be charged with intention and the working will be empowered by the extra time spent working on making your own product. A good option is also Tempeh because on top of being made from soy, it is also fermented which helps a connection to death energy since that is also what happens to corpses.
Speaking of fermentation, Kimchi or Kombucha are great options since they go through the process. Although fermented vegetables or any fermented vegan food is good for this, Kimchi is spicy and usually fermented for a long time before it is sold, so it’s an easy thing to pick up from the store and use.
For the bones you would collect in order to perform spells, rituals, or protection you can use driftwood or the resin bones that were never alive and are made from synthetic materials. Even dead plants, leaves, and tree barks work since they were once alive.
If you want to get really intense, you can invite a ghost or dead spirit inside of a doll or stuffed toy to interact with it. Of course this means it cannot be used by children and becomes a religious object. It must also be respected and in some cases salted or a barrier kept around it to keep the spirit inside from lashing out.
And as far as souls, it is possible to perform necromancy with the souls of plants and trees that have died, as well as the souls of moss, fungus, or other non-animal organisms. There are many coral ghosts because of the bleaching of coral reefs, and this provides much for a vegan necromancer to work with. It is also very easy to perform a vegan sacrifice since you would just take a dagger and cut open a watermelon or squash in a ritual circle to sacrifice it. Many spirits and gods also take dark chocolate since it has a similar content to human blood, and this is a traditional sacrifice to dark spirits such as demons alongside wine.