It's really hard to format a good title of the question, because it cannot be explained with just a few words.
An element with height of 100% and parent with fixed height in vw
(viewport width) is not actually 100% tall on Safari (Mac Mavericks, not sure about Windows). It's 0.
I've prepared a pen with example to demonstrate my issue. With Chrome the inner element has correct height of 100%. With Safari (v7.0.2 (9537.74.9)) the inner element is with 0 height.
Edit: The same bug occurs on iPhone with iOS 7.1