Vehicles belong to the group MissionCleanup.
Your best bet would be to cycle through the client's BrickGroup, checking for isObject(BrickGroup_1649.getObject(%i).vehicle).
For instance:
function getAllVehicles()
{
for(%a = 0; %a < ClientGroup.getCount(); %a++)
{
%client = ClientGroup.getObject(%a);
%brickGroup = %client.brickGroup;
for(%b = 0; %b < %brickGroup.getCount(); %b++)
{
%brick = %brickGroup.getObject(%b);
if(isObject(%brick.vehicle))
{
echo("Vehicle" SPC %brick.vehicle.getID() SPC "owned by" SPC %client.name);
}
}
}
}