If the buttercream looks broken just continue mixing and it should come together.
If the buttercream is loose and soupy place it into the fridge for 15-20 minutes before mixing again.
Substitute the vanilla for any other flavour you would like to use.
The buttercream can be stored in the fridge for 1 week or the freezer for 2-3 months. Let it come back up to room temperature and whip it again before using.