vim mode is generally OK but seems to be missing some commands. For example, vip, dip, do not work, whereas something like viw does work. All commands work in ace kitchen sink. Tested in RStudio 1.1.379.
RStudio doesn't have the very newest version of Ace so is missing some commands that have been implemented there since we last sync'ed (we've also added some commands; try :help in RStudio's Vim mode). We plan to bring in a new version of Ace in an upcoming release of RStudio.