Empty all of your vines onto the ground and leave 6 of one type. Test it and if it isn't strong enough, empty that type of vine from your inventory and add 6 of a different type. Keep trying until you find the right type of vine. When it says that your rope is strong enough, add one vine of a different kind to the your inventory and click the hill again. If it doesn't say that your rope is too weak, then add another vine of the same type. Keeping adding that type one-by-one until it says that your rope is too weak. If so, remove the last vine vine you added and add more of a different type of vine to your inventory. Keep up this process for the different types of vines until it is strong and long enough.