Add vertex without subdividing edge then vertex slide? I found that subdividing edges and vertex slide take away alot from my workflow.
Is it possible to not have to do the long process? Just create a vertex on desired part of edge directly?

I figured out it's possible to duplicate any vertex and snap it to an edge, but then the problem is how to merge this vertex with the edge. Is that even possible? 

Why doesn't knife tool work for me, when I want to create a vertex on an edge it doesn't cut only the edge, no vertex is created that way. Does it work for you guys? (knife tool doesn't create vertex on edge, if only the cut is for the edge, no face involved in the cut?)
The reason for asking because I want to optimize my workflow (don't waste my time with unnecessary process if there is a easier way to add vertices on edges where I want them)
First off you can use the knife tool to add only one vertex.
All you have to do is LMB  on one side of the edge, then LMB  on the other side, and press Enter or space to confirm.





There is another, potently faster, way of creating a new vertex. That is done by selecting a vertex on the end of the edge you want to subdivided and pressing AltD then G. That will run the Rip Edge tool then edge slide to move the vertex to where you want it to be. Because that tool automatically starts the translation on the new vertex you only have to press G once to get to vertex slide mode.





However using any of these methods will create Ngons, and in most cases really mess up your topology. So you should be very careful how and when you just add a new vertex in the middle of your mesh.