I wasn't aware of VTL; took a very quick look. It'd be a lot easier to comment if I could see some short examples, but I haven't yet found a "samples" section. It sounds like they really aren't playing in the same ballfield.
Their description says "It permits anyone to use the simple yet powerful template language to reference objects defined in Java code." As such it's designed to address a somewhat different class of problems than XSLT. Note that their Comparisons section says nothing about XSLT; they consider themselves to be more in JSP's class, a generic programming language for document generation from arbitary data rather than a rule-driven transformation language for XML data structures. So it sounds like "XSLT versus VTL" is an apples-versus-orangutans comparison. You can probably accomplish many of the same tasks with both tools -- or with straight Java coding against the Xerces parser, or with completely homebrew Java code, or with many other tools. The questions, as always, should be: 1) How well does the tool fit the task at hand? 2) How well does the tool fit the way you want to think about that task? 3) How important is portability of your solution, ability to share ideas with a wider community, etc?
