God may speak outside the pages of Scripture, because God can speak whenever and wherever He wants. But we should become increasingly humble about our claims the further we get from Scripture.
This would seem to be a very wise balance to strike between "my Bible is the only message" and "I had a vision in the night". God can and does speak through any means he chooses. But because the Bible is the only most clear and sure way he speaks to us, we must be have the humility to subject everything else to it. We need "holy scepticism”.
Wittmer is correct that humbleness is key. I think humility / humbleness is the correct word because it takes humility to admit that the vision may be just in our head. The pride of man is what drives many of us to claim things we have not experienced. And when we find that our "10 minutes to heaven" story does not line up well with the Bible we are prone to reject the Bible rather than accept our experience may not have been biblical, however, real it may feel.