Hmmm. When i remove the import, rmarkdown documents just print the HTML source of ggvis plots. If I add `library(htmltools)` to the Rmd document itself then it fixes the problem.
![image](https://cloud.githubusercontent.com/assets/129551/3031907/b3dc32f8-e051-11e3-9f3f-0fbf083f8471.png)