The problem with dragging down research in the slider is that it can grind research to a halt, allowing people with a stronger economy to leave you in the dust. You obviously cannot comprehend how commerce is related with research not only in Civ 4, but in real life as well. The United States is clearly would find a cure for cancer before India dispite India's advantage in population. If you make him too strong, he'll defect and will become an obstacle just like all the other civs. You're assuming that all Vassal State deals are formed by capitulation, which is entirely wrong.