Just take the train from Avignon Gare Centre Ville to Vitrolles. You can purchase tickets in advance, if you wish (16 euros one way, per adult in the rush hour times - around 14 outside of rush our periods). The tickets are good for any train departing from Avignon Centre Ville to Vitrolles in the same zone category booked (if you booked rush hour category, ticket is good for all rush hour trains - if you booked outside rush hour category, ticket is good for all non-rush hour trains). Or, you can purchase them just before departure, at the train station.
Book your train tickets to include the shuttle from Vitrolles to Marseille airport. You use the same train ticket to show the shuttle / navette driver you have paid for your shuttle transport.
Train seats are not reserved, but seat availability has never been a problem for us.
Once you arrive at Vitrolles, walk down the ramp (no other way) to the area where the buses stop (there are signs). Just wait for the airport shuttle to arrive (it is clearly noted on the shuttle bus). The shuttle takes you directly to the airport. From there, if you have a cell phone, you can call your hotel for a hotel shuttle pick up. They'll come and get you. Or, you can walk, depending upon which hotel you are at. I'd say the Best Western is the longest walk. The Ibis and Pullman are easier walks (due to the location and the traffic).
Most hotel shuttles begin transporting to the airport at 5h00. Be sure to get down to the parking lot around 4h45 as the shuttles do fill up. Not the end of the world if you have to wait for it to come back around and take a second load of passengers/bags, but for some folks, this makes them nervous, as they want to be AT the airport as soon as possible.
Edited: 9:35 am, August 06, 2012