Should I push back to the vendor and require them to respond with decent error messages if my XML is badly formed or typed, or is their position reasonable and I simply need to figure out how to validate my own messages?

Most of the people who write Web Services have no idea about interoperability -- they're using toolkits to generate the service, and couldn't change the error messages if they even wanted to.

Odds are, the stack traces they've giving back, besides being a potential security problem, actually have some clue as to where the problem is ... but you didn't have that, so I have no idea. If you can find out what SOAP toolkit they're using, you might be able to find some sort of support forum for it, and find some hints as to what the problem is.

As for pre-validating the XML -- the bulk of WSDL is just XML Schema, so you might be able to use an XML Schema validator for the payload.

Dealing with the 'impedence errors' that come from different SOAP toolkits is a royal pain -- make sure you're casting each element as a SOAP::Data object, or use a custom serializer -- the default SOAP::Lite serializer makes a lot of assumptions that can be false (eg, '1234' is an integer ... even though it's supposed to be a string in the WSDL).