In which way the velocity curve of Dorico (Playback Options) affects the velocity curve of a VST Instrument, for example, Pianoteq? What should I do to use only the velocity curve of the VST instrument?
It depends on how your VST uses velocity. A lot of them use it for dynamics, or to trigger dynamic layers, and a lot of playback templates are set up to make Dorico behave that way.
If its used for dynamics, the velocity curve that you see is generated by any of the dynamics(mf, mp, f, p crescendos, etc.) that you may have in the score. If you don’t want that, the best idea might be to not put any dynamics or dynamically related notations in the score - but you’d get a pretty bland performance I think. The idea is not to have to draw curves most of the time, but to use notation, - and as a last resort allow you to tweak them using the playback curves if there is just that one note or whatever that bothers you.
If velocity is used for something else, then you can use a different playback template to tell Dorico to do pretty much whatever you want. Still the same general idea though - you are using something in the score which is mapped to trigger whatever behavior you want in the VST.