The Lord knows what we go through, and from time to time will give us direct encouragement, as He does here with Paul. You cannot force God to give you a vision, but the more we walk with Him the more we will receive from Him.
Paul was going to face much opposition, but God had a purpose, to save many people. So Paul was to keep on preaching, and God assured him that no one would harm him. Note that elsewhere, both earlier and later, God speaks of the suffering Paul will have to endure. Sometimes God saves us from danger, at other times He takes us through danger. Sometimes there is suffering to be endured, at other times He will rescue us from suffering. There are no hard and fast rules on this. The key is to keep close to God and to follow the leading of the Holy Spirit.
Luke immediately gives an illustration of the vision being put into practice.Some Jews made a joint attack on Paul. Paul was about to defend himself, but there was no need. Gallio dismissed the charge out of hand, and it was the Jewish leaders who ended up being beaten.