Some may think it was very convenient for the church to make this vision of Peter's up to justify allowing the Gentiles in, but it is entirely consistent with what Jesus said in Matthew 15:11.
Just after receiving the vision, the men arrive. The Holy Spirit gives further instructions to Peter, ensuring that he does indeed meet them. We should not take this as meaning we should expect the Spirit to tell us every little thing, but this was a momentous occasion for Peter. We do not appreciate what a big thing it was for a Jew to treat a Gentile is equal, and equal in the eyes of God. What we can take from this is that if God does ask us to do very difficult things He will also provide all the spiritual support that we need, and will communicate with us.
The men relate the tale of what happened to Cornelius, and why they were at Peter's house. They also emphasise that Cornelius was a godly man, and respected by the Jewish people. Note that God did not make the situation impossibly difficult for Peter. Ie He did not choose a drunkard or sexually immoral Gentile. God does have some regard for us and our sensibilities, and usually takes us one step at a time, though sometimes they might be very big steps!