Also:

  Ctrl-w <    make vertical window narrower
  Ctrl-w >    make vertical window wider

(You can put a numeric count before the < or >)

Also:

  Ctrl-w =    make windows of equal size

And, when I want two vertical windows each of width 80:

  se co=161
  :vsplit
  Ctrl-w =

I gotta use 161 there because apparently one column is taken by the vertical separator.