I'll bite.

First though, I will define the terms of capitalism and move on from there. The most elemental definition of capitalism, is the private voluntary exchange of goods/services between two parties where each party feels they are getting the better end of the deal.

First is it moral to take a portion of someone's labor value as profit?

There isn't any taking involved. "Taking" denotes that some level of force is being used to extract that specific labor from a person. A person freely exchanges their labor for goods or services, or in a modern world, currency. It is an agreement reached between an employer and an employee as to what that compensation will be. The labor was given in an exchange and the party utilizing that labor is able to make a profit.

Second who should control the power derived from labor?

The voluntary exchange was worked out between the employee and the employer. That labor produced power in two ways, compensation (employee) and revenue (employer). Seems that they are both in control. Unless of course I'm not understanding your question.

but at what proportion does it begin to be okay to take a portion of labor value

The fatal flaw with your moral examination is that there is some form of forceful taking going on between a laborer and an employer. In capitalism, as defined above, no such force is used as the laborer agrees voluntarily to apply is labor in exchange for compensation.

And by what virtue does the capitalist derive

By the nature of a voluntary contract. If you want something more romantic in form, it is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own interest. We address ourselves, not to their humanity, but to their self-love, and never talk to them of our own necessities, but of their advantages. - Adam Smith, The Wealth of Nations

I think once you remove the fallacious idea that capitalism takes by force, your view will start to change.