NERDTree: открыть файл в текущем окне?

кажется, вы ищете вариант NERDTreeHijackNetrw Как поясняется в документация NERDTree:

If set to 1, doing a
:edit <some directory>

will open up a "secondary" NERD tree instead of a netrw in the target window.
Secondary NERD trees behaves slightly different from a regular trees in the
following respects:
1. 'o' will open the selected file in the same window as the tree,
replacing it.
2. you can have as many secondary tree as you want in the same tab.

если вы ищете что-то помимо того, что вы могли бы достичь его на основе API, предлагаемого плагином:

4. The NERD tree API *NERDTreeAPI*
The NERD tree script allows you to add custom key mappings and menu items via
a set of API calls. Any scripts that use this API should be placed in
~/.vim/nerdtree_plugin/ (*nix) or ~/vimfiles/nerdtree_plugin (windows).

The script exposes some prototype objects that can be used to manipulate the
tree and/or get information from it: 

g:NERDTreePath
g:NERDTreeDirNode
g:NERDTreeFileNode
g:NERDTreeBookmark

See the code/comments in NERD_tree.vim to find how to use these objects. The
following code conventions are used:
* class members start with a capital letter
* instance members start with a lower case letter
* private members start with an underscore

See this blog post for more details:
http://got-ravings.blogspot.com/2008/09/vim-pr0n-prototype-based-objects.html

одним из возможных решений является создание отображения, которое извлекает путь из файла под курсором и использует :edit на нем.