This leak confirms that HTC Vision and T-mobile G2 are one and the same phone. The leaked ROM can't be installed into any device yet, but the phone is going to appear in a month or so anyway, so this doesn't matter.
However, we can still get some information out of that file, because apparently it also contains a spreadsheet that says this that the HTC Vision is the G2, and that it will have Froyo at launch. There's also no mention of any customization of Android UI, and if you remember the G1 came with stock Android, so it's possible the G2 will have the stock Android version as well. You also have to consider that was the very first Android device, and HTC didn't get to make their Sense UI by then, so anything is possible.
There's also no mention if the rumors that HTC Vision will come with dual core CPU are true, but even if they are not it's still going to be a nice sequel to the G1.
You can find the ROM file here, but it's in Chinese so use Google Translate.