Are vegans racist since they say all lives matter instead of black lives matter?

if the slogan is used to mock #BLM then yes just like when the cops use #BlueLivesMatter.

I do plan on going back to veganism and yet still maintain support for BLM.

Not unless they say it in any way connected to "Black Lives Matter". Many people do feel that the lives of non-human animals matter very much, of course, and some of them certainly might have racist views.

What about the life of trees?

Haha

Not necessarily. But they are hypocrites when you consider the fact that vegetables are also living things.