Astronautical is right, make sure your vulpix has the pp left and moves ticked, otherwise maybe vulpix was stunned or slowed down, making it skip some turns.
try by giving your carnivoran buddy some rocks, see if it throws them.
lastly, check it's ability, maybe if it got hit by groudon and activated an ability to make it run away.
this is not the first time a confused question was asked regarding pmd sky, I also ready about the american version having glitches (just in case, make sure to name Pokemon in less than ten characters, even if there default name is more then ten characters, just in case) so maybe your game is being a little glitchy.
not much I can do to help, I have been replaying pmd explorers of time, which is pretty similar to sky and I didn't have trouble fighting groudon, if worse comes to worse and no solution comes your way, you can always bring a sleep seed or two and take groudon out yourself.
that is the best answer I have for now, hopefully it was helpful to some extent.
helping beyond time and darkness,