but would it really sound any different?
Yes.
The question is, to what degree and would it matter to you?
With a center like the VR12, with non-coincident mids and tweeters, the vertical displacement positioning (of the drivers, when the speaker is placed horizontally) would normally direct the main frequency dependent nulls/lobing (in the polar response) vertically. These nulls will be asymmetric, i.e., not the same above and below the speaker. Since perceptually, we are less sensitive to vertical anomalies in the soundfield than lateral, this is deemed acceptable in most circumstances, as we are generally seated spread laterally across the (surround) soundfield.
If you have the speakers currently with the tweeters both to the outside, or both to the inside, you probably won't notice the problems quite as much sitting centrally between them, as there will be some symmetry. You
should however, notice a significant change in the sound walking laterally across the room, as you move into the areas of the frequency dependent nulls.
The (further) problem with using one as center, placed vertically, is which side do you position the tweeter, left or right? The sound will change significantly to the left or right.
If you were determined to buy a third VR12, it would be better to place the center horizontally.
cheers,
AJ