The spotdiff.vim plugin will let you diff specific lines by visually selecting them (with V) and then using the diffthis command. If both ranges of lines are in the same buffer, then the plugin will create a new buffer to copy and paste the second range of line in that new buffer and make a difference between those two buffers just as Renaud Pacalet suggested doing, but the plugin does it for you. It seems similar to the Linediff plugin suggested by mfussenegger. The nice advantage of spotdiff.vim, for me at least, is that it overrides the builtin diffthis command, so it seems like just a nice upgrade without having to learn additional commands names.