Grass slides along the terrain? How to paint vertex alpha in the new blender version for unity?

https://youtu.be/DMS1I-gJkzE

According to google, making bottom vertecies opaque and top ones transparent should fix the sliding grass issue.

https://www.youtube.com/watch?v=1QbpCZuGe7M

I tried following t$$anonymous$$s tutorial, since blender recently got alpha paint feature, but I cant fix it. I tried making the bottom vertecies black and top w$$anonymous$$te, I tried making top vertecties black and bottom w$$anonymous$$te, I tried top vertecies transparent and bootm full opaque, I tried top vertecies full opaque and bottom vertecies full transparent, I tried top vertecies 1.0 weight and bottom vertecies 0.0 weight, I tried top vertecies 0.0 weight and bottom vertecies 1.0 weight.

Am I using it wrong? When I am painting the alpha, I can see the change in blener , I can see it get more transparent in vertex paint mode, but when I export it to unity, it still slides arround.