Those names make a lot more sense in relation to Pluto than Vulcan does. Vulcan was the God of Fire in Roman myth, Pluto was another name for Hades, the God of the Underworld. Kerberos and Styx are much more suitable.
William Shatner may be griping about this on Twitter, but if it had been named Vulcan I think a number of Trekkies would have been disappointed to see it's actually a small, irregular ice-ball. We should get a look at it in 2015 when the New Horizons space probe reaches Pluto.