The alcohol in vodka (or even better, Everclear) is a great solvent to pull the cocoa character out of the nibs, something that even a kick ass RIS can't do as well on its own.
As above, for a 5 gallon batch I put at least four ounces of nibs in a seal-able container and pour in enough alcohol to cover, seal it up and give it a shake now and then. Supplemental additions over the next few days to keep the nibs covered (they absorb quite a bit of the booze in the first day). Four or more days later the whole container gets dumped into a carboy through a sanitized funnel.
Just for something different, I'm trying a dark rum soak on a honey chocolate imperial stout right now. In my head it sounded right