As to the bible being vague on the life / treatment of slaves? Far from it. I do not endorse the following link 100% but it comes pretty darn close to demonstrating that the laws around slavery were intended to bring about human flourishing for all parties involved under the circumstances and opportunities of the given culture of that time.And if a man smite his servant, or his maid, with a rod, and he die under his hand; he shall be surely punished. Notwithstanding, if he continue a day or two, he shall not be punished: for he is his property.
In it, you will find that most slavery was for a limited time frame in order to get the indentured party back to self directed status.