4. If a person gets a hysterectomy or vasectomy for birth control purposes only, is this against God's will, considering the fact that our body is a vessel and he also said replenish the earth?

For Catholics, any form of birth control (except possibly the "Natural Family Planning" method) is considered against God's will and would be classified as sin. For Protestants, it is generally considered a matter of conscience (see Romans 14), since the Bible is silent on the issue. There is no place in the Bible that says one cannot use birth control (I would not agree that Onan's behavior in Genesis 38 is relevant to the question, since his sin came in failing to perform his duty -- in that culture -- of raising up an heir for his dead brother. By thus failing to produce an heir for his dead brother, he hoped to become the sole heir of his father's estate. Onan's sin, therefore, is essentially that of greed).

Personally, I disagree with the arguments commonly raised that to use something like birth control (or some other human intervention in nature, especially in biology) is to resist God's will. One could make the same arguments about anything that we as human's do, whether taking aspirin to relieve a headache, building a bridge to cross a river, or performing open-heart surgery. "If God had meant for us to...(fill in the blank)"

Human intervention in the natural order of things is not resisting God's will, since we are no less parts of his creation, and he doubtless has taken our behavior patterns (which he designed in the first place) into consideration in his plans. He made us in his image, after all, which would suggest he intended us to do godlike things.

So far as the statement to "fill the earth" means, that is rather general and I don't think that birth control has kept us from doing this, since the world population is presently larger than it has ever been in history. The population is growing principally because not so many people are dying anymore.