I have the following in my .vimrc alongside of neocomplete
set completeopt+=preview,longest,menuone
The preview window opens automatically for me as expected, however when I hit enter on a line of neocomplete or I start a parenthesis of a command (eg "foo.Bar(") the preview window closes and I have to remember all of the arguments that went with it.
I would like the preview window to stay open permanently or until I close foo.Bar()
:pcloseor<C-w>z. You will have to read neocomplete's doc again or ask on their issue tracker.