I have a vitamix, my vitamix can chop veggies on low speed, it can also mix the bars in thrive as well I guess it depends on the vitamix you have I ha e the 10in1. It have two different modes. I'd try it if I were you to see.
I have a high-speed blender (the Waring Xtreme which has about the same power as Vitamix), and I still use my food processor (though less often). It handles dry blending much better than my blender, which works best with wet blending. Most of the chopping/blending tasks I do in my food processor could probably be managed in the blender, but it would be much harder. (Also, I don't have a tamper with my blender, which makes certain tasks harder.) I also use my food processor for its grating and slicing capabilities, which no blender could ever duplicate. I can grate up a batch of cole slaw in no time. My food processor has a citrus juicer attachment which I use whenever I want to juice up a larger batch (for just one or two oranges or lemons, I use a hand juicer).
Blenders and food processors do overlap in their capabilities, but whether you are satisfied with one and not the other is a matter of your personal kitchen style.