I just checked, it doesn’t look like there is any special function in Inkscape to “convert to vector”. I guess it would be under “path”. In which case there is a “trace bitmap” option that didn’t work for me either.
And I got the PNG to show up in the parts editor, but not the breadboard.
I understand that having SVG is the nicest way… But I’m serious when I say I don’t want to draw the entire board. If I have to, I’ll simply draw the two connector pieces on a blank PCB - which is more than enough for my purpose.