I have a .tex file which attempts to \includegraphics with a .pdf file.
This is perfectly possible when explicitly running pdflatex but doesn't work with latexmk because latexmk runs with latex and not pdflatex. (This SE answer pointed me to the latex vs. pdflatex problem.)
I'm interested in getting my compilation to work with latexmk because I'm using the Vim plugin vimtex. Is there a way?

latexmkruns withlatexrather thanpdflatex. But if you runlatexmkwith the argument-pdf, it should compile withpdflatex. – Sverre May 11 '15 at 12:05