If you adjust the vertical alignment it will affect the horizontal, you can't avoid it. You are correct that you need to turn the inner part of the optical wedge, found under the bezel. Be careful to turn the INNER part, not the outer part. It'll probably be difficult to turn, use some tweezers or similar, and soak the periphery with alcohol for 10-20 mins if it is. This will soften the shellac used to glue the inner ring.
If the vertical isn't far out it may not affect the horizontal enough to worry about. If you do need to adjust it then adjust the infinity first. Take off the screw just below the cyrillic "E" on the front. Behind that is a tiny screw for adjusting infinity. Make sure you use a target some distance away (try a streetlamp pole in the far distance or something - not one in the next street!).
Again, you may just get away without adjusting close-focus if it wasn't far off in the first place. If that's not the case then you need to adjust the wedge-shaped RF sensor tip after removing the lens. Turn it with long-nose pliers and don't turn it much at a time, it's quite sensitive. Now the fun starts because you will have upset the infinity setting. It's a case of adjusting infinity & close focus, a little at a time, until they're both correct. Tedious and very fiddly but not difficult. Note which way you move the settings so you can be sure you're not getting further away!
By the way, congratulations on the FED - they're nice!
