I heard an anecdotal story recently that one of the reasons for the building of the Death Star was that the Emperor was aware of the coming Yuuzhan Vong invasion and wanted a superweapon for defense against them.
This seems rather suspect, so I'm wondering if there is any evidence from canon to either confirm or deny this. Sources from main canon and EU are preferred, though not required.
Note that the Yuuzahn Vong are entirely part of the non-canon Legends continuity; they do not exist in the new canon. So in the "main canon", the answer is clearly no: Palpatine did not anticipate the Yuuzahn Vong, because the Yuuzahn Vong do not exist1.
In Legends, the answer is
The novel Outbound Flight reveals that, at least as early as 27 BBY, Palpatine was aware of a threat of invasion from a group referred to as the "Far Outsiders":
"There's an invasion coming," he said. "A massive assault force of dark ships, shadowy figures, and weapons of great power, based on organic technology of a sort we've never seen before. We believe these Far Outsiders, as we call them, already have a foothold at the far edge of the galaxy, and even now have scouting parties seeking information on worlds and peoples to conquer."
[...]
"When will Darth Sidious tell [the Republic]?"
"When he's turned the Republic's chaos into order," Doriana said. "When we've built an army and a fleet capable of dealing with the threat. To announce it before then would do nothing but create panic and leave us open to disaster."
Outbound Flight Chapter 18
The Far Outsiders sound very much like the Vong we meet later, but they're confirmed as one and the same through Vergere, who's said in Rogue Planet to have left Zonama Sekot with the Outsiders:
"We came here to buy a ship from you, and to find out what happened to Vergere."
Shappa chuckled grimly. "It's all tangled, isn't it? She's gone."
"Where did she go?"
"She left with the Far Outsiders."
Rogue Planet Chapter 45
She turns up again in the New Jedi Order series as a servant of the Yuuzhan Vong.
Although we don't have confirmation that the Death Star was itself intended to be used against the Vong, we know that Palpatine was:
Which seems to pretty strongly suggest that the Death Star was a tool for use against the Vong, even if only indirectly.
1 At time of writing, anyway; the Lucasfilm Story Group hasn't shied away from borrowing popular EU elements for new canon works, so you never know how things might change in the future. And note that some newer canon novels (Empire's End, Thrawn) indicate that Palpatine was concerned about threats in Dark Space, and that the Chiss Ascendancy was monitoring hostile somethings out there, so it's not impossible that we could see the Vong return to canon
Grand Moff Tarkin, the Death Star's chief proponent and architect, didn't think so.
Tarkin is one of several viewpoint characters in the Legends novel Death Star, which covers the eponymous station during its late construction and testing up to its destruction at Yavin.
Throughout the novel, Tarkin discusses and monologues about the purpose of the Death Star and its role in the Empire. He views its worth primarily in terms of the Tarkin Doctrine, that "fear will keep the independent systems in line - fear of this battle station", as it puts it in A New Hope. He believes that making the consequences of rebellion direct and immediate - destroying the perpetrators' planets, their communities, bases, and loved ones, all at a single stroke - will produce more fear than conventional firepower.
At various points Tarkin discusses the Death Star with "traditional" fleet captains and commanders, who seem to think it's a waste of resources - formidable yes, but less capable in a direct ship-to-ship fight than the (presumably large) number of warships that could have been built with the same resources. Tarkin doesn't really seem to dispute this point, but he believes that winning battles is secondary to spreading fear among the rebels and winning the war - which he believes the Death Star can do more effectively. He doesn't seem to think the Empire will be involved in the kind of ship-to-ship battles that a fleet of Star Destroyers would be preferable for. Nor do any members of his staff (or Darth Vader, who is supervising) seem to think this will be a priority.
In other words, in his mind, Tarkin thinks of the Death Star as a tool for defeating a widely dispersed, numerous, but ill-equipped popular insurgency, not for fighting a belligerent empire with a massive navy.
