[Konstantinos]

By this command the vehicle isn't specified.
Use sscanf.

pawn Код:

CMD:v( playerid, params[ ] )
{
    new vehicleName[ 64 ], vehicle;
    if( !sscanf( params, "s[64]", vehicleName ) )
    {
        vehicle = GetVehicleModelIDFromName( vehicleName );
        if( vehicle < 400 || vehicle > 611 ) return SendClientMessage( playerid, l_red, "Veicolo non trovato!" );
        new Float:a, Float:x, Float:y, Float:z, String[ 128 ];
        GetPlayerFacingAngle( playerid, a );
        GetPlayerPos( playerid, x, y, z );
        new PlayersVehicle = CreateVehicle( vehicle, x, y, z, a + 90, -1, -1, -1 );
        LinkVehicleToInterior( PlayersVehicle, GetPlayerInterior( playerid ) );
        format( String, sizeof( String ), "Hai spawnato: %s", aVehicleNames[ vehicle - 400 ] );
        SendClientMessage( playerid, l_green, String );
    }
    return 1;
}

Also, about this I am not really sure

pawn Код:

if(IsPlayerInAnyVehicle(playerid) == 1)