You might be surprised.
Previously, game designer Shinji Mikami revealed that Vanquish was initially inspired by Casshern. I can see that as there is a definite Casshern vibe going on.
Now? Now he has revealed that the game's backgrounds have been influenced by the so-so medieval romance Tristan & Isolde.
Yeah, I can't see that at all. Here's the quote from Mikami: "The backgrounds are influenced by a movie called Tristan + Isolde. In the end, the visuals are really the product of the individual tastes of the staff creating them, but I think the graphics in the game are something we can be proud to recommend to people."
The influence certainly is not obvious, but apparently it exists.
Vanquish Fan Interview [Platinum Games]