Iona Jacqueline Wilson said at one point that readers could decide that. In my opinion Jade has a mental health problem of some kind and is imagining Vicky. I also read an essay on a theory that Vicky was never real in the first place.
Melissini Alexiadis I think that Vicky is a ghost, but only people she was really close to can see her. At her funeral, when everyone was at Vicky's house, her mum says to Jade she see's her too.
♫✯Em loves Hollenstein✯♫❤the summertime and butterflies all belong to your creation❤