How do I find out what my Apple vendor ID is?
This number may be a bit harder to find since Apple re-designed iTunes Connect on Mon, Feb 17th, 2014. 

Multiple vendors: If you go into the monthly financial section and have multiple vendors a drop down will show up in the upper left with this number. 

If you only have one vendor: Download a monthly financial report. The vendor id is the long number at the beginning of the report name before the first underscore.