[Kostas']

There is nothing like

pawn Код:

if(!IsAdmin(playerid, 1))

And

pawn Код:

PlayerName(playerid)

at Jordiee's code. I suggest you to look the code before post something (code)
Sarma93's code with some changes.

pawn Код:

CMD:vrespawn(playerid, params[])
{
    if (!IsPlayerAdmin(playerid)) return SendClientMessage(playerid, -1, "You are not admin!");
    new
        bool:vehicleused[MAX_VEHICLES];
    for (new i=0; i < MAX_PLAYERS; i++) {
        if(IsPlayerConnected(i) && IsPlayerInAnyVehicle(i)) {
            vehicleused[GetPlayerVehicleID(i)] = true;
        }
    }
    for (new i=1; i < MAX_VEHICLES; i++) {
        if(!vehicleused[i]) {
            SetVehicleToRespawn(i);
        }
    }
    return 1;
}