This is my initial port of the modifier (T55089).

However, I did notice that even before my port, that the modifier is kinda broken in 2.8. Mainly two things:

1. There is no feedback in the viewport. Previously you could see the paint in solid mode.
2. Painting to vertex colors seems to clear each frame. So the paint doesn't stay. (Haven't tried texture mode)

This is still the case after my porting work. So I'm wondering if this is something that needs fixing within the modifier or if there are other factors at play here.