I know she has returned but is she staying or is it just a small guest return like Grant?

She seems to be a regular. I'm not sure how long she's signed up for. Belinda was a regular but didn't stick around long

Yes she is a regular and i think we will see Ned Porteous back as Mark Jnr as well as Vicki.

I wonder if Scarlett Johnson will return as Vicki in the future?

I haven't seen her on TV since she was in Beaver Falls in 2012.