I've found a way to get the 3-column window layout with vc-dir, vc-diff and vc-log I showed in my previous post about SVN workflow in Emacs by default, without the manual setup I was complaining about.
(Update: It looks like this customization only exists in Emacs 23.)
The key is that vc-mode uses a standard function,
pop-to-buffer, to create the new window for vc-diff and vc-log. This is the function that keeps creating horizontal splits that make no sense on my widescreen display. However, it ends up calling a function called
split-window-sensibly, which can be customized (of course!). So, if I set the "Split Height Threshold" to nil,
split-window-sensibly will never split horizontally, and then the default behavior just magically does what I want. If I start with just a single vc-dir window and invoke vc-diff and vc-log, I get my three columns!
To get to the right customization screen, just do
M-x customize-apropos <ret> ^Split.*Threshold. That'll get you the two relevant customization items. Enjoy!