I think it's an error to say that schematron has low vendor support, considering that there is a schematron implementation written using xsl-t it follows that the vendor support is very high, same goes for examplotron.
In fact all the schema languages which I have mentioned have a high level of support since they come with open source implementations which you can embed in your applications.
What I meant is that their support embedded in applications such as XML editors or browsers (and even in other specifications such as XPath 2.0) is currently weaker than the support of DTDs and W3C XML Schema.