Hi Lisa thank you so much and I am thrilled to hear you thoroughly enjoyed this course! Great question ref making the room sprays. I'm actually going to edit the text a bit on that lesson - I should have added a bit more detail as to how to do this. You're right that you don't use any base oils, just the essential oils into the witch hazel or water. I will add to the lesson though that this will only last a week or two max as there is no preservative for the water. 
You can indeed use vodka, this will act as a solubulizer for the essential oils (will help them mix in water) and help preserve it too so it will last longer. However it won't make it last as long as proper grain alcohol (like perfumers use) as the alcohol percentage won't be high enough. But if you're happy to just keep it for a week or two (make a small amount say 50ml) you could try a mix of vodka, water, and your essential oils on top. Always give the bottle a shake to make sure the essential oils are all dispersed in the mix before spraying. And avoid your finest furniture just in case! Here's a great article by the Tisserand Institute if you'd like to read more. Thanks again Lisa! https://tisserandinstitute.org/effective-use-alcohol-aromatic-blending/