I've been playing as England, Rome and Persia on Earth18 (difficulty Emperor), and in all my games China, and Russia are my greatest threats. In Medieval and later ages , they often Vassel like Crazy and gang up on me. Even if I've built up a massive Empire, their mega stacks wear me down.
In my most recent game , playing as Persia, as soon as I had Immortals , I sent two up to Moscow , and destroyed it , before going after closer enemies, this looks like it's a winning strategy, however when playing as Rome (slower UU), or England (further away), it's going to be more difficult to take out one of the big 2 early on.
I have two questions :-
a) can anyone recommend a strategy for England (or Rome) to avoid the late game Russia, China axis of death
b) Vasselage in Earth 18 seems very silly at times, in one of my games, Catherine managed to Vassel Washington, after DOW on him and taking just one of his cities, why can't I do that ?, does anyone play Earth 18 with Vassle'ing turned off ?
China always seems to be strong, probably because it has great land and is surrounded by relatively weak neighbors. There isn't much you can do about China unless you're playing an Asian civ. Russia, on the other hand, doesn't seem to be as frequently threatening. Sometimes it is, sometimes it's not.
So as for your two questions:
a) If you're playing as a European civ, Russia is usually fairly easy to take out. Rush everyone else in Europe first (Izzy, Louis, Freddy, etc.) and tech your way to catapults. I usually hit Cathy with a stack of swords/cats, no matter who I'm playing. Usually this is right around the time Cathy starts to switch into expansion mode, so she will have lots of recently settled cities that are poorly defended.
Except if you're Rome (see below).
China, you can't really do anything about; they're just too far away. You can either try to attack them late-game (generally difficult and it could a long war, but doable), or focus on dominating everyone else and simply ignoring them.
b) America is usually very weak because Monty almost always declares on Roosevelt and takes his land. That's probably Roosevelt was so quick to capitulate. The Americas are usually behind in tech, so it shouldn't be hard to take cities from them.
Now, if you're Rome, you don't need to worry about anything. You can conquer at least Spain, France, England, Germany, Greece, Russia, Arabia, Egypt, and maybe Persia with Praets alone (depending on difficulty). And you should be fine from there. Just beeline Iron Working to get Praets ASAP.
Neal's King of the World #16 is pretty helpful.