After a chat with a Viewsonic tech and pulling out a lot of screws, this is what I discovered you have to do;

Find a secure place to turn your VG930M upside down, like on an armchair.

Undo the central screw on the base and remove the rotating black plastic plate.  Make sure you know which way it goes back on again.

Remove all the screws on the metal plate, something like eight around the circumference and another two which attach to the LCD support column. (You were never just going to pull that base off!).

Now for the tricky bit! The support column has two extensions that extend down into the base and click into position.  You have to get a flat blade screwdriver and push one of them towards the centre while trying to lift the base just a little away from the support column, so that it is unclipped.

Then you have to do the same with the other extension, while lifting the base a little. The base should now pop off.

Not as straight forward as some screens but achievable!
Good Luck!