Was it correct to use 25612-21212.vps.digitalpacific.com.au at those steps?
Yes, that's fine, in fact it is preferable. I always leave the hosting-provider-assigned hostname as the primary hostname, in case anything unexpected should happen with the vanity domain's DNS records.
Okay, I think I see the problem here. Tempted to "LOL"...
As I mentioned in a previous post, Postfix will complain if a given domain is used for both myhostname and mydestination in Postfix's main.cf. Now that you have removed the VPS hostname from the mydestination directive, you need to add the mail domain 25612-21212.vps.digitalpacific.com.au via the ISPConfig interface and create a mailbox or a catch-all for it.
The part I failed to realize is that you are attempting to send mail to @25612-21212.vps.digitalpacific.com.au, and not @urbancampsite.com, as evidenced by this excerpt from your log file:
To be clear, I think you had two different problems that were "stacking-up". We resolved one of them, but then you changed the test-case (not your fault), which made the second problem harder to spot.
Try making the above change and then report back when you have a chance.