For Deltat there is actually sometimes availability MIA-LAX so that's good...but unfortunately it's only in coach, rarely in business, but it's only 5 hours.
I think you'll actually have more trouble finding the availability on that with a DL ticket than to SYD on VA. Another alternative if VA isn't available or you want to try other options you can also try CZ MIA-LAX-CAN-SYD or KE via ATL, LAX or any of their other gateways. It really all depends on your dates.