I have the following vector plot and I would like to add an vector inset to the plot.
How to make the inset layer as transparent or any relevant solution?
Seemingly your grey shape has a big white background rectangle. Try to remove it or raise the curves to top.
If the grey shape must be on top, then take the pencil and draw a clipping path around it. Select both and goto Object > Clip > Set. This is the only way in Inkscape, if the grey shape happens to be a bitmap image.
Another possiblity is to try to ungroup the grey shape and to delete the unwanted parts.