From digging around in the source code it looks lite it should be possible to invoke the validator warning and not throwing errors for the vendor extensions by feeding it this line of code:

java -jar css-validator.jar --vextwarning=true http://url

And it doesn't choke on the argument. But these to lines produce no different output:


# java -jar css-validator.jar --vextwarning=true --output=xhtml http://url

# java -jar css-validator.jar --vextwarning=false --output=xhtml http://url


Further on it is not visible or documented in:
java -jar css-validator.jar --help