Generally speaking, you need horizontal dropouts for internal-geared hubs, to prevent the axle from rotating. It's not a matter of chain tension. There are some very few hubs (like one of the several versions of the Rohloff) that can be used with vert drops, though I only know of the Speedhub. The other option open to you is to work something out with some inventiveness, spare parts and maybe a power drill that will hold things securely in place. Good luck, if you decide to go that route. It's been done, but it will require some ingenuity.
Vertical dropouts - great for derailer bikes, enormous pain in the fanny for anything else