The D12 is quite a motor but I'm not sure it will lift a Vapor that has been modified to a 29mm mount and with a dual deploy av bay with two chutes.
Not meaning to derail the thread, but what I'm about to write can be adapted to the Vapor as well as the Olympus.
AtomicStorm, you are going to run into the same issue as the Op with the weight issue, This is what I would suggest... Because the Olympus is an odd size buy two kits and build one stock except put the aft centering ring 1 inch from the end. Put the middle ring at 2 1/4. Take the second kit and build it per the instructions right up until you get to the step to attach the launch lug. Instead of attaching a launch lug, cut the body tube to somewhere around 5 inches. Now you need a coupler and you can use the plastic tube adapter for that. Cut it at 3/4" above the raised ring and 7/8" below the ring. Scuff the bottom 7/8" up real well and epoxy into the top of the booster. Sand the top 3/4" if needed to it slips into the back of the upper stage. There you go with a two stage Olympus.
Added: Concerning sanding, the fit should be snug enough that it doesn't fall away without an engine installed but does if an engine is installed
I would also suggest papering the booster fins for strength since it is tumble recovery
-Bob