In the remarks on this page, it explicitly says that you can't update a portion of a constant buffer, but there is no mention of doing it for a vertex buffer (although it mentions updating vertex buffers in general, so it should be possible).  I've never explicitly tried it, but I would assume that you use the same addressing concept for the destination box as you would for a 1D texture (there is also a sample of that on the link above).

It would probably be worth doing a quick test, where you create a vertex buffer that is all zeros, then update only the second element with all ones.  If it works in that case (which can be verified with PIX) then you should be good to go.

As a side note, the two methods are similar enough that it shouldn't be too hard to build support for either method and dynamically decide which one to use.  This would be ideal so that you can profile and see how fast one or the other is.  In the end, they both end up doing the same thing - copying data from CPU memory to a resource in GPU memory.  There is some difference in the way that the driver handles it, but I suspect there isn't much performance difference between the two methods.  To be honest, I've never heard anyone say that one is better than the other...

And one final point - notice the comments about deferred contexts at the bottom of the linked page!

EDIT: I also found this link, which has some good discussion about the topic.  In there it says that mapping a buffer uses bytes in the D3D11_BOX, while textures use pixels for the units...

In the remarks on this page, it explicitly says that you can't update a portion of a constant buffer, but there is no mention of doing it for a vertex buffer (although it mentions updating vertex buffers in general, so it should be possible).  I've never explicitly tried it, but I would assume that you use the same addressing concept for the destination box as you would for a 1D texture (there is also a sample of that on the link above).

It would probably be worth doing a quick test, where you create a vertex buffer that is all zeros, then update only the second element with all ones.  If it works in that case (which can be verified with PIX) then you should be good to go.

As a side note, the two methods are similar enough that it shouldn't be too hard to build support for either method and dynamically decide which one to use.  This would be ideal so that you can profile and see how fast one or the other is.  In the end, they both end up doing the same thing - copying data from CPU memory to a resource in GPU memory.  There is some difference in the way that the driver handles it, but I suspect there isn't much performance difference between the two methods.  To be honest, I've never heard anyone say that one is better than the other...

And one final point - notice the comments about deferred contexts at the bottom of the linked page!