Even as Hannah prayed and yes you are correct, she kept her vows, this is important. The bible tell us not to get into vowing let it be yes or no since you are very correct in saying that some people forget their vows after receiving the request. When we pray, in the name of Jesus, even if we know not how to pray as is said in Romans 8:26, the Spirit maketh intercession for us so that God can hear and answer our prayers according to his will which is always best for us. Blessings.