SOLVED!
I'm using these BOB from steppers online. It has a built in low pass filter that was the cause of this slow up ramp. I have now used a normal 5v output pin and added my own low pass filter. I've setup the VFD analogue input to 5v and its working perfectly.
The new version of the plug in triggered a compatibility warning and based on the above info it obviously didn't fix my issue, so I've rolled it back.