I got the Drake Sword, went to the Altar, and sure enough, there was the vagrant. For a while I watched the little bugger walk around and swipe at me with its claws, until I eventually killed it and received 1 humanity. I've since checked the wiki to learn that the type I encountered was the white evil vagrant.
It was very cool to come back to Dark Souls and encounter something new, even if it did make me think about what a missed opportunity the vagrants are in terms of enemy design - the concept behind them is great, but it would have made for a much more interesting and exciting encounter if the vagrant had actually been a dangerous and more visually impressive enemy. We'll see if DSII delivers anything along these lines.
I'm on my third created character and have racked up nearly 200 hours of playtime in total, so I'm pretty pleased with having encountered a vagrant, given I've read posts stating how some players have played for a lot more hours than this and still haven't encountered one.