I would like to request a new eventHandler that fires locally on the machine where it is attached to a unit (in this example the player itself) when ever the unit changes vehicle.

This eventHandler would be very useful, because we could get rid of infinite loops like:
waitUntil{vehicle player != player};
And vice versa, only to check if a player has entered a vehicle.

The eventHandler could then return the following:
0 = Object the eventHandler is attached to.
1 = Vehicle that the unit was in.
2 = Vehicle that the unit has entered.

Example:
player addEventHandler["vehicleChanged",{ hint str _this }];

Result if the player enters a vehicle:
"[BIS_fnc_objectVar_obj1, BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d]"

Result if the player leaves the vehicle:
"[BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d, BIS_fnc_objectVar_obj1]"

It would be perfect if the eventHandler would also fire, when the unit changes its position inside a vehicle and return the current turret or assignedVehicleRole with the eventHandler!

Example which returns "assignedVehicleRole":
player addEventHandler["vehicleChanged",{ hint str _this }];

Result if the player enters a vehicle as driver:
"[BIS_fnc_objectVar_obj1, BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d, ["driver"]]"

Result if the player changes seat to passenger:
"[46328100# 5: mrap_01_unarmed_f.p3d, 46328100# 5: mrap_01_unarmed_f.p3d, ["cargo"]]"

Result if the player leaves the vehicle:
"[46328100# 5: mrap_01_unarmed_f.p3d, BIS_fnc_objectVar_obj1, ["cargo"]]"

The assignedVehicleRole does not change if a player leaves a vehicle (even thou he becomes the "driver" of the units body), so one would have to check within a script or function inside the eventHandlers Code, if the player has entered or left a vehicle by using (vehicle player == player) or (_this select 1 == player).

Therefore it would be better to have a more complex return as I will describe in the next part:

Another, more complex example, with a bit of scripting (which could be integrated inside the eventHandler code, to make it run native == faster):

player addEventHandler["vehicleChanged",{
_this select 0 Unit the eventHandler is attached to.
_this select 1 Previous unit vehicle.
_this select 2 Current unit vehicle.
_this select 3 (fullCrew [vehicle (_this select 0), ((assignedVehicleRole (_this select 0) select 0)]) select 0)select 1
}];

Result if the player enters a vehicle as driver:
[BIS_fnc_objectVar_obj1, BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d, [BIS_fnc_objectVar_obj1,"driver",-1,[],false]]

Result if the player changes seat to passenger:
[BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d, 46328100# 5: mrap_01_unarmed_f.p3d, [BIS_fnc_objectVar_obj1,"cargo",0,[],false]]

Result if the player leaves the vehicle:
[BIS_fnc_objectVar_obj1, 46328100# 5: mrap_01_unarmed_f.p3d, BIS_fnc_objectVar_obj1, (Nil or empty string "")]

In the end, the most perfect eventHandler that offers the most usability for scripters would look like this:

_unit addEventHandler["vehicleChanged",{
_this select 0 Unit the eventHandler is attached to.
_this select 1 Previous unit vehicle.
_this select 2 Current unit vehicle.
_this select 3 assignedVehicleRole (also returns "driver" correctly if player leaves a vehicle!).
_this select 4 // Turret or empty String
}];

This eventHandler would be extremely helpful in certain scenarios and could help to improve client performance by getting rid of many useless loops.