Thanks to misuno_caitlin I have been watching Ergo Proxy on YouTube (provided by Funimation, even!) and Vincent Law's voice just sounded... familiar.
And this time, I got it right! Liam O'Brien is both the voice of (English dub) Vincent Law and Niall in Dragon Age: Origins.
I feel better now.
EDIT: Interesting credit way down at the end World of Warcraft: The Burning Crusade - Illidan Stormrage Things that make me go 'hmmmm'.