[Scenario]

I did this in like 2 minutes;

pawn Код:

command(vdive, playerid, params[])
{
    if(sscanf(params, "d", value))
        return SendClientMessage(playerid, COLOR_WHITE, "SYNTAX: /vdive [height]");
    if(!IsPlayerInAnyVehicle(playerid))
        return SendClientMessage(playerid, COLOR_RED, "ERROR: You aren't in a vehicle!");
    new Float:Pos[3], value;
    new vehid = GetPlayerVehicleID(playerid);
    GetPlayerPos(playerid, Pos[0], Pos[1], Pos[2]);
    SetPlayerPos(playerid, Pos[0], Pos[1], Pos[2]+value);
    SetVehiclePos(vehid, Pos[0], Pos[1], Pos[2]+value);
    PutPlayerInVehicle(playerid, vehid, 0);
    return 1;
}

It uses ZCMD and SSCANF.