The only time I've used cocoa powder I put it right into the mash. I used about 500g in a 10 gallon batch of imperial stout. It worked just fine giving the beer just the right amount of chocolate flavour, not at all overwhelming but noticeable.
I can't comment on the oak chips as I've never used them but the vanilla you're fine for more than 7 days. I did a Neapolitan Stout (cocoa, vanilla, raspberry) and let the vanilla beans soak in an ounce or so of The Balvenie Scotch. Mind you, it was only one bean, scraped and chopped. Added the whole thing to the 5 gallons in secondary for a chunk of time, more than 7 days. Works perfectly! Will be doing the same with a pumpkin spiced beer–adding the vanilla to secondary. I do think one bean was noticeable but in retrospect, two might have been better. It's there but only at the end along with the warming alcohol.
One thing that might be interesting with your recipe would be to do a smaller beer with the second runnings. The last couple of imperial stouts I did we got the volume that we wanted for the imperial portion and then kept lautering to do a smaller volume of lower gravity beer. This is sort of the opposite ratio for when you do an actual partigyle brew (it's usually a larger volume of lower gravity and a smaller volume of high gravity) but it works just as well and creates a very nice small beer.