Only he doesn't.
If you read Vaz's comments it is clear that he doesn't want the referendum to be about the treaty at all. He told the BBC:
"I am absolutely convinced that we will win any test of public opinion as to whether or not the British people want us in Europe, at the heart of Europe, which is what's happened over the last 10 years, or whether they want us to turn our back on Europe."In other words he wants a question along the lines of "Do you support our beloved leader Gordon Brown's ceaseless efforts to secure a golden age of prosperity by placing Britain at the heart of Europe and resist the blandishments of those who wish to bring about a new dark age of poverty and rampant paedophilia?"
This is an attempt to dodge the question of the reform treaty. It is an attempt to prevent what may well be the dominant view in Britain - that we support European cooperation but are sceptical about the idea of closer integration - even being expressed.
So I am still happy to call for a referendum on the reform treaty.