The only problem was it was completely dead, checking the easy stuff first the fuse in the plug and the one inside were both fine. I could tell right away from the missing and mixture of screws that someone had already been inside before but luckily they seemed to give up when they saw the fuse was fine and they didn't damage anything.
Checking the output of the transformer and it was completely dead, a bit of research online and this is quite a common issue. I had a look for a replacement but the one maplin used to sell which is known to be an ideal replacement they no longer stock. I found another on ebay that was a good match but rated for 4amps where as the original is rated for 3amps (or possibly 2?) I was worried it would be too big and require some case modification that I'd prefer to avoid.
While continuing my search I found this very informative video here https://www.youtube.com/watch?v=zIqCVLveSys&t=1322s which shows how to repair the original transformer. It seemed that my transformer might be fine and the only issue is a bad thermal fuse hidden in the windings. You don't have to dig too far into the transformer to bypass or replace it either.
Here's my teardown, bypass of the fuse and rebuild.
I sealed it all back up and fitted / soldered it back into the vectrex it fired up first time with no other issues.
Thinking about it afterwards I didn't have to tear into the transformer as far as I did, with the metal cover removed from the 18v side the two wires for the fuse are visible at the edge, I could have just soldered a wire or old capacitor leg across them or if I wanted to be fancy a new thermal fuse.
The thermal fuse really isn't needed though, there's already two fuses on the 240v side.
So there you have it, repair your old transformer don't replace it and a big thank you to Hugo B. on youtube. I don't think anyone knew about the hidden fuse until his video, I certainly didn't. :)