On the subject of VistA and the web of data, here's some background about SMART, a project to enable "substitutable" apps that run on multiple electronic health records systems, personal health records, data mining platforms, and health information exchanges. We define a RESTful API to expose patient-level data using W3C's Resource Description Framework -- and the WorldVistA team has built a prototype capable of running SMART Apps running on VistA!
SMART WorldVistA at NHS Hack Day 2012
Developer documentation of the API, including our RDF data models
Background about SMART