Lexis, Vocabulary, Lexeme, Lexical item - what's the difference?

What's the difference between lexis, vocabulary, lexical items and lexemes?

The finer distinctions may be arguable, but take a look at the diagram created below:

The difference between, vocabulary, lexemes, lexical items and lexis - a diagram

You can also have a look at the short treatments in Wikipedia on the Lexeme and the Lexical Item.

Searching with Google brings up a very succinct treatment of Vocabulary vs. Lexis on the website of Stephen van Vlack who teaches at the Graduate School of TESOL at Sookmyung Women's University, Seoul.