Over two hours later, the 22 GB Steam download is still going. I don't think it ever hit double-digit MB/s again. It's been hovering around the 2-3 MB/s range.
My friends are - as always - are also not experiencing any of these issues on separate providers.
I called Vodafone, and it unfortunately wasn't very productive. They're wanting to send a new modem and have a technician set it up, but I don't think much will come of it.
If there's no improvement, I'll just deal with it and hope that Chorus doesn't take too long.