[G2 and Desire Z Q&A, Help & Troubleshooting] wheeliest:
So where is the missing 2GB?

To answer this, it is first necessary to understand a bit about how internal flash cards, such as the Sandisk card (also known as an "emmc") on the HTC Vision, store information.

Luckily, Sandisk has provided a helpful video. Pay particular attention to the discussion of SLC (single-level cell) and MLC (multi-level cell) in Chapter 5.

In an SLC configuration, a single bit is packed into each memory cell. In a multi-level cell, you can fit 2, 3, 4 or more bits in each cell. You get a lot of capacity with more bits per cell, but at the expense of speed and reliability.

In trying to figure out where the missing 2gb went, scotty2 noticed that most of the Sandisk card's block device (that is, the part with Android on it) had been partitioned within the regular "User Data Area" as an "Enhanced User Data Area".

Note that when we talk about partitioning the emmc, we're not talking about regular MBR partitions like /dev/whatever. An emmc partition is a very low-level partition of the flash. Each emmc partition constitutes a full block device, which can then be further partitioned into a bootloader, /system, /data, etc.

The card's datasheet wasn't too clear about what the "Enhanced User Data Area" did that was so different from the regular User Data Area, although one thing was clear-- once its parameters was set, you couldn't "un-set" it. To quote the datasheet, the Enhanced User Data Area "can be programmed only once during the device life-cycle (one-time programmable)."

But why was the entire Sandisk partitioned in this special "Enhanced" User Data area? No one knew.

Then tmzt found this. It's an article by Toshiba that suggests what's going on:

Those areas requiring better reliability are SLC or can be programmed as SLC. . . the Enhanced User Data Area, which may store, for example, system log files, are SLC. The User Data Area, which may store music, pictures, videos and other files is MLC. . . Each 1 bit configured as SLC results in 2 bits less of MLC. Theoretically an 8GB e-MMC device (densities are defined in MLC terms), could be configured virtually all as SLC and thus would be approximately 4GB. In most cases, it is more likely that the majority of the memory would be configured as MLC to support higher density.

You've probably figured out by now what's likely happened here. Assuming the Sandisk emmc works like Toshiba's, the 4GB flash has probably been, save for a few tiny partitions such as the radio, irreversibly configured to use SLC, rather than MLC. If so, the benefit is faster performance and perhaps greater stability (and more read/write cycles). But its capacity/density would be cut in half.

And that, my friends, may very well be where your 2GB has gone.

[edit] So To Conclude...

Assuming the above is a correct understanding of the issue, the following appears to be the case:

The HTC vision has a 4GB firmware card

It has been irreversibly partitioned to use a faster/more reliable configuration called SLC

This has resulted in a practical capacity/density of ~2GB

Update: Initial investigations from over a month ago reported that T-Mobile attributed this issue to "creative partitioning": ("I called into T-Mobile Android support and was assured this number is correct, and that I do have the full 4GB storage on-board... there's just some "creative partitioning" going on.") This may correlate with the explanation provided above.