Creation is Gamebryo.
Gamebryo's shaders got an update for the night lighting on New Vegas strip, it was updated again in Skyrim (forget why) after which Bethesda renamed it to Creation, but otherwise they are one and the same engine.
The code is the same.
Also regarding New Vegas, Bethesda didn't develop it, Obsidian did.
Bethesda had nothing to do with the game except licensing the engine. Which is why fundamental parts like storyline, voice acting, character development and general feel aren't anything like Fallout 3.
Obsidian care about quality, contrary to Bethesda.
This is why I sighed at the beginning of the comment.
I have a simple task for you, should you believe that you are correct enough in that statement. Prove it.
Here's what I've been able to dredge up, relating to the engine used for 3, New Vegas, and Skyrim.
Gamebryo is an engine which can be licensed by different developers.
3 and New Vegas both proudly tout that Gamebryo was their development engine.
Gamebryo takes a licensing fee for the use of their engine.
The Creation engine was cited and being used in Skyrim.
Based upon these facts we know:
1) If Gamebryo was being utilized to run Skyrim there would have been a licensing fee paid to them.
2) Bethesda said they were tired of the poor graphical fidelity of Gamebryo, which is why they were going with the Creation engine for Skyrim.
3) There are no pending suits against Bethesda from Gamebryo.
4) There is an established history of law suits for stealing engines (read: the Silicon Knights fiasco with Too Human and the X-men game).
So in order to prove that Gamebryo is Creation you'd need something like a creator's tool kit...oh wait, they released that to the public. Well, you'd need a game that was a financial success to make legal action viable. Oh wait, Skyrim took 2011 by storm and printed money. So with easily obtainable proof of infringement, and a big juicy target, why wouldn't Gamebryo sue Bethesda for their actions... I'll wait while you mount the logical defense of "well, the commands are the same." While you do that, allow me to postulate.
If it takes a person 10 hours to learn a thing, and they use said thing for 5 years, it will take more than 10 hours for them to unlearn it and learn something new. This is why manufacturing is reluctant to replace old machines, and why new machines often forego logical placement of things should there be a long established status quo. Likewise, code monkeys do better if their instruction set doesn't dramatically change. Having basic instructions change might be logical, but if you've type a line 10,000 times then changing what you type will be annoying and make you prone to failures.
Back now, let's see. You can prove the console commands are the same, yet Gamebryo hasn't pursued a lawsuit. We all understand that code can perform complex features in any number of ways (read: linking the frame rate to physics), so even if an engine was completely rewritten it code use the exact same console commands to do the same things. Heck, let's just throw all that out the window. You've obviously proven Gamebryo=Creation, so why don't you go to Gamebryo's creators and suggest they sue Bethesda. Given that the game is raking in money, along with enough cross-promotional crap (read: beer, pop, and everything else Bethesda is shilling), you should be able to sue them for enough royalties to never have to work another day in your life.
I was on your side of the argument once. Problem is, it just doesn't track through. Unless Gamebryo wouldn't defend their property, you're stuck with zero logical reason as to why they aren't suing Bethesda. So we're clear, it doesn't matter that Obsidian developed New Vegas. Bethesda dictated terms, as they were the publisher. It's hard to say, but they pulled an EA here. Develop the game to our specifications, or we'll find somebody else. Obsidian did. Objectively, I'd say much better than the Fallout 3 developers did. They, to my standards, did better with the same tools and justified why I am always willing to buy an Obsidian game early (while Bethesda's in-house games are a wait and see due to the burning I received from pre-ordering Skyrim).