Anyway, after all that ordeal, I decided to go back to my initial proposition of vacuum forming and to create a light fixture instead of a vase. I would also try to cast in the vacuum formed plastic as a test to see if it would work.
My exterior blue foam form had somehow survived the many processes I had put it through and Andrew, being amazing helped me route another interior piece. I then lathered the pieces in vaseline and put them into the vac former. It took a while to get the pieces out but I was pretty happy with the pieces. I vac formed 2 interior and 2 exterior pieces, cut them out, and sanded the edges clean so that they were aligned. I also put nuts and bolts in to hold them together. I consciously chose to make those a feature of the light piece as opposed to a clean seal with another adhesive as I liked the grungy feel of the bolts.
I then vacuum formed another 2 pieces which I used as moulds to set the resin in (as shown in the last image in the above montage). This time it set pretty well and I was happy that my initial concept of the 2 textures worked and looked really good when I put a light through the back.