If you are using it as a second primary you should be fine, the c02 from ferment will fill the extra space, if it going into a secondary you want to be careful in your racking and time it sits as it will allow head room for oxygen to hang out and slowly creep on yer brew.
Another thought..often the whiskey is added at bottling..you can soak your vanilla beans in the whisky beforehand and just add it to half the batch when you bottle.
I do more with less than most do at all!
Look at all the Everything!
It's my pleasure to Meat You!