How would I bind a key to open up a buffer or file in helm in a new vertical or horizontal split? (BTW, I'm using spacemacs).
With ido I use the handy ido-other-window package to open existing buffers or files in the ido list to new window splits. For example, instead of pressing RET to open in a new window C-2 the buffer/file will be opened in a vertical split.
In the Helm popup, you can open a buffer in a new window using F3 or C-co.
Press C-z for the full list of possible actions.