You can know that thought that with a certain pitch and power setting eg 15deg nose up and max power that the aircraft will be climbing at Vx.
Now you are correct though that if you were in a downdraft which was more than the climb rate at Vx you would be going down not up. But there ain't alot you can do about that if you are a VX anyway and max power.
The angle of attack of the wing in normal flight will be directly related to pitch. Unless we are talking about gradients for terrain seperation or approach it isn't normal to try and achieve a set gradient. You may want a certain climb rate to achieve a gradient which will depend on your ground speed. The setting an attitude and power equals performance will set up your aircraft to achive rates of climb, speeds etc as if you were in still wind.
If you start wanting to relate that to using the ground as your reference you will have to do some mental calculations to figure out what you actually need to get your required gradient. You can use timings or 1 in 60 rule to figure it out.