Palpatine and Thrawn and all of them only knew that something dangerous existed outside of the galaxy and was eventually poised to attack. They did not know WHEN they'd attack, HOW they'd attack, WHERE they would begin their attack, WHAT the goal of their attack was, and what the strength of the attack force would be. They predicted an attack would be within the near future (read: before 50 ABY, probably) and they knew that they were dangerous. They knew nothing else. Thus, they would have been caught by surprise
if the full force of the Yuuzhan Vong decided to unleash itself on Tarkin's systems and wipe out his Death Star before he even got to scare the Rebel Alliance with it and let Skywalker blow it up.
And he was hardly well-defended against them. Palpatine never created any wide-spread defense system to ward off any extra-galactic attack.
And, again, I make reference to the fact that various factions actually played both sides or sided with the Vong. Any ambitious individual, wickedly greedy individual, and people who really
hated Palpatine (like, say, one of those persons whose family was wiped out by the Empire or something similar to the histories of many Rebels) may have seen fit to try strike a deal with the Vong to 1.) Try keep their planets safe and 2.) Remove Palpatine from power to possible 3.) Gain more power for themselves.
Now, Mon Mothma would have realized it was a choice between two evils and, yes, probably would have spoken solidarity with the Empire against the Vong. However, the Rebellion would never have sided
with Palpatine's Empire. "The enemy of my enemy is my friend" wouldn't exactly apply. The hatred of Palpatine and the atrocities he committed were so great in the eyes of the Rebels that most would never have approved any sort of formal agreement with them-- likewise, the Emperor and hard-line Imperials would have never agreed to "helping eachother out" like the Imperial Remnant and New Republic did after Bastion was sacked. The two sides would probably have unofficially agreed to stop firing on each other for the time being and protect themselves from the Vong.
And what new military and political strength would Palpatine get? His armies would have already held more numbers than those of the Vong. He was already the great dictator of the galaxy; in A New Hope he had disbanded the Imperial Senate and completely moved control of the star systems into the hands of his puppets, the Moffs. His power was already absolute. If you disagreed with him or if he didn't like you, you were declared a "rebel" and an enemy of the state and executed anywhere from one to ten times. Creating the first Death Star was to scare away the growing rebel sentiments across the galaxy and enforce a "rule by fear" galaxy-wide. The Second Death Star was to now counter the ever-growing power of the Rebel Alliance, though some will say (and it is possibly true) that he was creating the Second not only to help eradicate the Rebels, but also to defend against a possible Vong invasion.
Prince Xizor and Black Sun, knowing them, may have possibly attempted striking a deal with the Yuuzhan Vong. Xizor would probably be more than happy to attempt to hand over Darth Vader to the Vong--as he was always looking for a way to get rid of his rival, and vice versa--and probably would have seen this opportunity to get rid of Palpatine and gain strength himself. He most likely would have been like the Hutts-- playing both sides. However, his final motives would have probably been to strike a deal with the Vong and betray Palpatine and Vader. He perhaps might have been successful with Vader, but--as I have said in other posts--I sincerely doubt Palpatine would have been killed, and would have likely gotten rid of Xizor and Black Sun for the betrayal. i'm just saying, Black Sun works for money and power, and some huge alien force wiping out the Black Prince's rival and removing the only man in the galaxy man powerful than he probably would have been tempting.
---In fact, during the Yuuzhan Vong War, the disorganized and slightly weakened Black Sun did not cease attacks against the Jedi Order and the New Republic.
And, as I have mentioned in other posts, extremely ambitious Imperials and radical Rebels may have formed Peace Brigade-like troops to support the Vong and increase their own power and chances of survival.
Go read up on 'em. Formed almost exactly after the war began. Not just a "few" people were in it. They didn't realize that the Vong would not grant them mercy until much later in the war, and even after the Galaxy realized that mercy from the Vong was pretty impossible, the Peace Brigade still managed to hang on, with people like Senator Pwoe--who attempted to seize the Chief of State position after the Fall of Coruscant--tried to sign a peace treaty with the Peace Brigade (only to be arrested when New Republic forces captured the Peace Brigade's capital, Ylesia).
Now, with so many Imperials still holding considerable power and so many Rebels really, desperately wanting Palpatine gone (note: as I side, the "big ones" like Mothma, Leia, etc. would probably not advocate siding with the Vong), it is quite likely that a more powerful version of the Peace Brigade would emerge, with several ambitious Imperial warlords/moffs/admirals/etc. rising to try assert their own power.
As I said, when the Empire was victorious, those who joined the Vong would probably all be executed, but the Empire would not be stronger-- it would, in fact, be weaker, and the threat of full rebellion might be greater if the Vong threat was eradicated. Someone could easily move to take out Palpatine and put someone else in charge of the Empire, and maybe--just in case the Vong attacked again or something--work out some deal with the rebel forces (Speculation; a post-Vong Empire/Rebellion thing could really go in multiple directions).
Yeah, yeah. If only it were a Krath Debate.
Note: Since I can't write long posts for the Run-On anymore, YOU get to bear the brunt of my weekly long post. So nyah. >:)