I’m flying to Vietnam from Vanuatu and I have a stop in Sydney, Australia. The luggage are not directly sent to Vietnam so I have to take them out in Australia and send them again. In that case, does it require for a transit visa ? Btw I’m only staying for 3 hours.
Timatic, the database which airlines use to verify passengers' travel documents, states in part:
Visa required, except for Nationals of Vanuatu transiting through Cairns (CNS) or Sydney (SYD) with a confirmed onward ticket for a flight to a third country within 8 hours on the same calendar day. They must stay in the international transit area of the airport and have documents required for the next destination.
If you are able to stay in the international area, then you won't need a visa. However, you said that you have to recheck your luggage. In this case you have to go through immigration in order to get your luggage, so you do need a visa.
You also should consider that for separate tickets, three hours is probably too short a connection. You are likely to miss check-in your connecting flight depending on how much time you spend waiting in immigration. You have to have your baggage checked in to the connecting flight an hour before departure, and that might not be possible.