Two Star Destroyers can defeat it. It is the bigger brother of the Miid Ro'ik. The Miid Ro'ik can defeat a mark 1 or 2 Imperator in one-on-one, but it will be extremly damaged afterwards. A Mark 3 or 4 Imperator will defeat one in one-on-one. The Uro-Ik V'alh can take an Imperator in one-on-one will heavy damage, but still quite space-worthy. The Mark 4 Imperator is the only Imperator model that can kill it.
Janus, the Executer would waste it. It could probably take at least three of them by itself and still not have any outrageous damage. If the Imperisals still had the Eclipse, well, the Vong would be screwed.
The biggest things that Yuuzhan Vong have a Worldships. Those are heavily armed, but they aren't dedicated warships. They are generally weaker than the Executor, but their sizes are extremly different from ship to ship. One of them was 120 kilometers in diameter. Two thirds as big as the first Deathstar. The rest were closer to 10 kilometers.
There was also the Legacy of Tourment which was the Yuuzhan Vong's largest dedicated warship. I think there was only one of them. It was about equal to the Executor.