Separate... ànd keep the faces?

This is the problem: I want to do proportional vertex editing, but I only want a part of my mesh to be affected by it. I know, you can scale the area in which it has effect, but my mesh is pretty complicated and still I want to use propverted over a large area.
[>] So… is there a way to separate a part of the mesh, but still keep all the faces between the 2 objects you get after separating?
That way, I can separate, without ruïning the complexity of my mesh, do the propverted on one of the 2 objects and then rejoin the 2 objects into one mesh that looks a lOt nicer :smiley:

Thank you very much!

Just hide the vertice you don’t want to affect.

H to hide the selected vertice
Shift-H to hide everything except selected
Alt-H to reveal hidden vertice