So in RFC how do you go about making other civs you . .. .. .. .. .? What strategy do you employ and suggest for others?
Vassals are great. If you get one who offers without needing to be bashed into capitulation is the best. You can demand of these guys for ever (at least in my experience). Capitulated vassals can turn and declare war on you though when you demand something they don't want to give up - I've had this happen to me a couple of times.
I will always accept a vassal when they have access to different resources that I can't otherwise get easily myself. Why trade for stuff when you can demand stuff? I will generally deny a vassal who crawls in my direction if they have bascially the same resources as me. Especially given the new restriction on how many vassals you can have (4), getting a poor vassal is not a great idea.
I almost always vassalise the Inca. They can get much more value out of the mountains in South America than anyone else can with their unique ability. They are inherently more likley to vassalise than the Aztecs for instance also, I think there is a value which causes this in the leader personalites.
A vassal which will always accept gifted cities over a large region is a good vassal, as you can conquere other colonies and pass them over to your vassal to avoid (some) of the upkeep costs and stability issues. Inca (South America), Mali (Africa), Khmer (Australia), Aztecs (North America) are the best examples and often the 4 vassals that I aim for if I'm going for an empire which might be able to win a domination victory.
Sometimes when attacking the Aztecs, they refuse to offer Capitulation as an option even when their last city has just one troop left in it. I've found that sometimes the reason for this stubborn behaviour can be that there are actually a bunch of Aztec units wandering around somewhere in North or Central America which appear to give the Aztec leader the impression that he should hold out a bit longer. If you track down and destroy these wanderers though, I have found that the Aztecs then decide to offer Capitulation when otherwise it wasn't in the diplomacy menu.
Don't forget to gift back any conquered cites to their original owners after they capitulate to you. I usually gift the Pikemen units (and sometimes the catapults too) which spawn from the conqueror events to the natives after they vassalize.