use flags.
For each vehicle/group to reach the "goal" zone, set a specific individual flag.
Set up a constant trigger to look for "Flag is true" for each vehicle combination..ie "if flag 5 is true and flag 6 is true and if flag 7 is true" then "mission = success".
It will require quite a few trigger's to be set, but basic logic can be set up in this way.
To cut down on combinations, I would use 4 or 5 groups of vehicles, and assign a Flag to the success of each group.
If you assume 4 groups, then that is 16 possible combinations in total, of which 5 = a 75% success rate (1111, 0111, 1011, 1101,1110) and since "1111" is covered in the other 4, thats ony 4 constant triggers to test for ie:
if (flag1 AND flag2 AND flag3) = mission success
if (flag2 AND flag3 AND flag4) = mission success
if (flag1 AND flag3 and flag4) = .....
and so on
not perfect , as of course "Group in Zone" will be true whether it is 1 vehicle from the group, or all vehicles in the group, but still...extrapolate using bit logic (which is all flag values are used for), and go from there.
and good luck!
(it would be nice to have some more basic logic setup in the ME to test for! )