Ornamec 170 probably won't do anything to vinca. Ornamec is a GRASS herbicide
and is labeled for "over the top" applications for grass control. "Over the
top" means you can spray it over many different perennials and ornamentals to
control GRASS.

If you still have the container, you should see that it plainly states "Grass
herbicide."

When using any chemical or fertilizer, remember to "READ THE LABEL" to make
sure what you are using is labeled for the purpose you need.