Considering his background, I don't think he was talking about algorithms. In the past he became involved in this project:
"Quantum computing (QC) is a flexible model of computation and there are many physical means of implementing it. One obvious means of implementation is by classical simulation. In other words, performing QC by running ‘quantum software’ on conventional hardware."
[1] https://web.archive.org/web/20131005014920/http://noospheer....
I take this tweet as Vitalik saying that a similar thing could happen for mining (or general search), and while it would be surprising, it doesn't seem like it is impossible.