Headspace and vanilla addition

I currently have an imperial stout fermenting. I wanted to experiment with this batch so I split the 5 gallon batch into 3x one gallon jugs and left the ~2.5 remaining gallons in a 6.5 gallon brew bucket.

Now to my questions:

1. Is 2.5 gallons in a 6.5 gallon bucket too much headspace? My original thought was that such a big beer would be able to fill that space with CO2 but now I am wondering.

2. This is supposed to be a vanilla imperial stout. Assuming I haven't already created an oxygen problem, how can I add the vanilla without risking oxygenating the beer? I'm thinking if I open the primary after two weeks 4 gallons off oxygen are going to fill in. Should I:

A. Wait the instructed 2 weeks and move the 2.5 gallons to some 3 gallon container with vanilla? This seems like the most obvious approach but involves me buying a 3 gallon container.
B. Leave out the vanilla addition for all around fear of oxygenation?
C. Add the vanilla early (say 1 week into fermentation) so that the oxygen I introduce can be pushed back out during the more active stages of fermentation?

Thanks in advance for your thoughts/advice.

 

As long as the bulk of the fermentation is finished I would go ahead and transer the 2.5 gallons to a 3 gallon with vanilla. You can always repurpose the 3 gallon fermentor for bulk aging different beers.