No. XML is case-sensitive.
Well, if you really have to, you can do
*[lower-case(name()) = 'vendor']/text()
in XPath 2.0.
But there's something badly wrong with your XML document design if you have this requirement.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference