The Vibrant's PowerVR SGX 540 GPU is still going to be king. The G2 will perform great in graphics, however you must keep in mind it has a smaller screen, this is how the G2 will be able to compete with the powerful sgx540.
What? No. The Vibrant and the G2 have exactly the same resolution, 480x800. A physically smaller screen just means a higher DPI and a more compact phone. Other than that, most of what you say is indeed helpful.
(Disclaimer; I've pre-ordered the G2)
The G2 is heavier, yes, but that's primarily because of its metal unibody construction, as opposed to the Vibrant's super-light plastic.
The screens are the G2's Super LCD vs the Vibrant's Super AMOLED. The Vibrant looks nicer, with deeper contrast and more vivid colors. However, the Super LCD is reportedly a little sharper (objective) and a lot more power-efficient (up to 5x more, I hear).
The 800mhz Scorpion chipset outperforms the Hummingbird currently, but a fair comparison isn't really possible yet because the Vibrant is still on Android 2.1, while the G2 comes with 2.2. Also, the Vibrant has a GPU, so all told, the two phones are about equal.
The power savings the G2 has from the screen and lower clock speed are partially counteracted by its smaller battery: the G2 has 1300mAh, while the Vibrant has 1500.
Now let's get into the G2's real advantages. The obvious one is the keyboard. The Vibrant has Swype, so it's almost competitive with an actual keyboard for typing words. For typing things that aren't words (code, gaming controls, URLs, app shortcuts, etc.) a physical keyboard is a million times better. The G2's keyboard is supposed to be one of the best there is, too: the unique Z-hinge design means that the keyboard is level with the screen, so there are no "hard-to-reach" top row keys.
Many people seem to not care about the optical trackpad for some reason, but I definitely prefer to not have to obstruct the screen with my huge fat fingers to do mundane tasks like scrolling or selecting. I'm in the minority there, though, it seems.
HSPA+ is not inconsequential by any stretch of the imagination. Current 3G on T-Mobile gets 4mbps up to 7mbps, whereas HSPA+ can get speeds up to 14mbps. You do need to live in or around one of the >50 big cities that T-Mobile has set it up in to see immediate results, but they're wheeling it out to more places all the time.
One last thing to note, the G2 runs vanilla Android, whereas the Vibrant has Touchwiz. Practically, this means that the G2 will get updates as Google/T-Mobile release them, whereas with the Vibrant you need to wait for the updates to go through Samsung. (This is why the Vibrant is still on 2.1). This isn't anything a little hacking and modding won't solve, if you're up for it.
Near as I can tell, most other features (camera, GPS, wifi, neither have noise-celcellation) are the same. So it comes down to:
Vibrant: 33% Lighter weight, larger and brighter/more vivid screen, Swype.
G2: Keyboard and Touchpad, metal construction, HSPA+, more power-efficient, vanilla Android.
You already know which one I chose.
You can compare specs directly
here.