Checking through is largely irrelevant because you have to collect your bags at SYD and then re-check after customs and immigration (similar to US arrivals).
If you have DL status, doesn't matter how you bought the VA ticket as long as your DL number is attached -- GM and above should entitle you to an extra bag on VA.
"*Complimentary checked baggage allowances for Diamond and Platinum Medallion members will be equivalent to a Velocity Platinum member. Allowances for Gold and Silver Medallion members will be equivalent to Gold and Silver Velocity members. Fees may apply for overweight and oversized baggage. Please visit Virgin Australia for more details on baggage policy."