Vincentio?

This may sound like a stupid question... but how do we know that the Duke is called Vincentio? I only discovered this today as i was researching on the net. We have been through the whole play in detail and never come across this name...

Thanks!

In my copy of Measure for Measure (Arden Edition), there is a list of Dramatis Personae which lists VINCENTIO, the duke
Hope this helps, I seem to remember there was something on Wikipedia about some copies not saying that his name is Vincentio.