I go for vassals in almost all my wars. Beeline their capital, capturing cities on the way if necessary, and sometimes they will capitulate before you get there. I then give back all their cities. Then on to the next until I am secure. Recently I discovered that keeping one city allows the building of heroes so I may change that part of my general plan. As to accepting weak nations as my vassals. It depends, if they are not at war and on my border, or a vassals, then yes. Otherwise the decision is more difficult. In one game I conquered a single city civ on a plateau island and both the other civs on that island, both single cities, offered to be my vassals. I accepted. Now I attack and capture the barbarian cities first just in case the ai has been waiting for such a scenario to vassal to me. The Topkapi Palace wonder is about strengthening your vassals, by giving them some gold out of nothing. It needs changing, perhaps to give you access their culture so you can get the heroes.