Firstly I presume you booked online.

I have seen where DL.com will not allow the booking more than one domestic VA segment at a time while there is availability over two. Generally you do need to call Skymiles to have this done.

Secondly, if you have two bookings - VA-xVA then it is likely they will check your bags through - just have both e-tickets with you on check-in in Melbourne.

To collect bags and recheck at SYD Domestic, I would allow no less than 1½ hours between flights.