This section of the Psalm ends with a plea by God to the Israelites to continue their sacrifices (in the Old Testament), but offer with a heart that is broken and seeking Him. In the New Testament, we look to Jesus as fulfilling all of the Old Testament sacrifices, but the point is the same. God longed for the people to follow their offering with thanks and gratefulness to Him for His grace and forgiveness to them.
This vow of commitment comes with a promise in verse 15. In effect, the writer is saying that if we have a heart for God and are willing to make a difference for Him, we can call upon Him when we are in trouble and He will deliver us.
Does this mean that God only helps those who are fully committed to Him? The answer is no. Since God is God, He may do what He wants and when He wants even to those who lack faith Him. God. Sometimes He just does miracles in order to get people to follow Him or draw closer to Him in the first place.
What about for us committed Christians? May we trust in God's deliverance? I would say yes, but one has to remember that God promises to rescue us "His way" and on His timing. He is there guiding us through our tough situations, but He only promises to see us through them, not necessarily to rescue us from them.
The point of these two verses is that it is never a waste of time to commit our lives serving God. Not only do we get blessed by helping others, but we can then count on God being there for us when we really need Him because we do have that "heart" for Him.