Do you think the dealer just forgot to order it?
Possibly - ask to see the dealer order records. What explanation have they given you ?
When I ordered mine I left a £500 deposit and 2 weeks later when the order/spec had been confirmed by the factory, I paid a further £500. Got my car after 8 weeks and had to sell my Boxster earlier than anticipated
If they have cocked up your order I would be seeking some compensation. You can always walk away, but then you are back to square one. Alternatively, some dealers get an extra build slot due to cancelled orders or increased production - if you order a new car now you will probably still get it by July

If you go down this route, I would argue for a (better) discount at the outset though. One other thing here - if, at any time after placing your initial order, you change the original spec, your order goes back to the standard build time again - you haven't been this naughty have you

You may find that the car does arrive earlier but 6 months is a big wait overall as is mid July from now. Try ringing VW Customer Services and argue that if they cannot expedite the build, you will cancel the order. I'm sure a bit of hard nosed bargaining will get you somewhere

Good luck and keep us posted.
Cass