What you can do - and if you're comfortable doing this sort of thing...
I see two choices:
1. Disconnect the speaker connections from the cube speaker. Attach some length of speaker cable (MUST BE SPEAKER CABLE, NOT AN INSTRUMENT CABLE - you can use "lamp" wire) and run that to the speaker in the Vox (disconnect the electronics from the Vox speakers.
2. Be even more creative and flexible.
a. Disconnect the speaker out on the Vox from the speakers and connect the Vox speaker to a female 1/4" cabinet jack and turn the Vox into a "true" extension cab. Then do the same thing for your cube.
b. Connect the speaker out cable inside the cube to a 1/4" male plug (again use speaker wire to add some the needed length).
With #2, you can plug the cube back into itself or into the Vox. Plus down the road, you can use your cube as an extension cab with some other Frankenstein set up you come up with. But based on my personal experience with both of those - I bet you'd be pretty happy using the Vox as an extension cab for your cube.
BTW - I'm saying all of this assuming that both speakers are the same impedance!!!!!! If not, then you have some other considerations to consider
Good luck! You might try to get your Vox up and running though.