Hi Carsten,
I don't have experience with Vivenza, as Corlan has recommended, so don't want to suggest that PS is the only or best way. But here's a way that I would do it, pre-supposing certain image characteristics, etc., etc:
Open the image in PS...
Make adjustments to curves, hue/saturation, dodge/burn, etc. These adjustments would be on separate layers in my case. Opacity control of each layer is of significant value, so working in layers is much better than using "Image -> Adjustments -> whatever...
Click on "Background" layer in the layers palette. Duplicate this layer. Select this layer by clicking on it in the layers palette (probably already selected). Go to Image -> Adjustments... and modify hue/saturation or use any other adjustment appropriate to make the sky look correct. Ignore the other areas of the image.
Confirm that the "background copy" layer is selected in the Layers Palette. Click the Layer Mask icon at the bottom to create a mask. Choose the Brush tool (command-B). Set the opacity of the brush to 100% if you want to block out the changes to all areas other than the sky. Press the letter D to set the brush to "default" (black and white, not some other funky color). Black blocks out stuff in the layer, white allows it through. "X" toggles between black and white as the brush color. Make sure your brush color is black. (Using 100% brush opacity in this example, but might prefer several passes at a lower setting.)
Paint the areas that you don't want to have received the color adjustment (the landscape not affiliated with the sky). This leaves only the sky to show your color tweaks.
Once finished, you can then adjust the opacity of this layer (the background copy layer) to fine-tune the appearance.
Wow, that's a lot of rambling crap. There are some gaps there, and some blatherings. A bit much for a single post, esp. without screen shots. Sorry.
There are lots of online tutorials, as well as books for this, but I'll have to think about where the best links may be. To be honest, this is quite simple if we were sitting together, but a bit messy to write out without visuals.
One could also do this by other techniques, such as working on a duplicate image (rather than a layer) and then bring that in on a new layer, etc. But the above method is pretty straight forward and can produce good results.
Dale
Edit: you can control some of the smoothness of the transition b/n layers by choosing a softer brush. e.g. you might try a brush hardness of 35%. This, together with layer opacity, will provide a smooth transition.
Edit 2: if you prefer to PAINT IN the changes to the sky, you would want the initial mask to block out the layer (the background copy on which you tweaked the sky). You would then option-click the layer mask icon to add a mask which was filled with 100% black (or fill it with black from the edit menu). Then with all of your changes to this copied background layer blocked by the black fill, you would paint in the sky with a WHITE brush. The choice is usually dictated by the amount of work to hide or show the respective elements of the scene.
NOTE: the layer mask must be selected in the layers palette for the brush to function properly. IOW you want the MASK to have the border showing, not layer icon. This is the normal state if you add a mask, but if you find things not responding correctly, confirm that the mask is actually selected.