How accurate is the validation?

vatnode queries the official VIES database in real time, so accuracy is as good as the EU system itself: when VIES says "valid", the number was registered and active at the exact moment of the check. Some member states update their registrations closer to real time than others, so a very recent change may take a day or two to appear. Valid results are cached for 15 minutes for speed; for absolute certainty on a high-value transaction you can force a fresh VIES check with force=true.