OK! I am getting ready to ice my cake before I ganache it but I have run out of vanilla! Can I leave it out or not?

if you are talking about the icign yes you can leave it out, or add another flavor. if you leave it out, you may need to add a little more water or milk to replace the liqued.

So I could add like butter rum or butterscotch or peppermint oil flavoring.... something like that?

As long as it will compliment the ganache's flavour, go with it.

Thank you all! I went with a drop of butter rum and added milk in place of the vanilla. It turned out very good!

You can take the required amount of liquid and dissolve vanilla sugar if you want the vanilla flavour.
Also, check your pantry for vanilla syrup, like Torani.
They (Torani) make great flavours (meant to be shots in cofee) which I would imagine could be great! Be careful though, they can affect consistency....