This is the best answer I've come across so far, considering the experience I've had with vegetable stock and broth.....stock is way more concentrated

Whether there's a difference in home cooked aside, if you mean supermarket veg broth/stock (sold in cans or boxes,) I don't think there's a difference - just a marketing choice re what they call it.

Make no bones about it, there is a qualitative difference between vegetable broth and vegetable stock. Broth uses clean, well trimmed vegetables - prepared like those you'd plan to consume in a chunky vegetable soup - while a stock uses clean, untrimmed vegetables with leaves, stems, stalks, roots, shoots, skins, and peels - or sometimes just the trimmings saved from the preparation of other dishes. It may also be important to note that while a vegetable broth may be very delicious as is, without dilution, a stock will have more concentration of flavor and SHOULD be diluted. Vegetable stock is primarily used AS an ingredient while vegetable broth IS the result. It would be sensible to make vegetable broth from stock, but it would not be sensible to reverse the process to make vegetable stock from broth.

I found this method interesting for making a veggie stock. And using Agar Agar to clarify and replace some of the 'mouth feel' you would get from bones.

http://veganfooddelivery.com/vegan-living-in-the-21st-century/83/

Stock usually means the use of bones, like veal or chicken stock, to make an intense liquid base you probably wouldn't eat on it's own. I think a broth (veg, or say, chicken) would be a clear simple soup, ready to eat. So, really, the prep for a vegetable 'stock' and a veg broth wouldn't be that different, since the richness and gelatin from the bones wouldn't enter in...
Clear as...stock, right!

Stock is usually the building block and broth the the final product, but for vegetables I can't see much difference. Plus general misusage would obscure what the recipe was really calling for.

Here is a wonderful vegetable broth from Anne Thomas which she goes on to use for soup.
http://www.projo.com/food/content/fd-love_soup_review_01-13-10_8OH2R53_v6.1fc7b48.html

I don't think there's a difference for veggies. Typically when you make stock you boil the bones with the veggies, etc... Obviously with a vegetable based stock/broth there are no bones to use.