How do I avoid botulism formation when vacuum sealing and freezing raw chopped vegetables?
I suppose dropping the bags in liquid nitrogen for a few minutes, and then storing in the freezer might suffice.

mostly fish, but not a bad read:
http://www.fda.gov/downloads/Food/GuidanceRegulation/UCM252416.pdf

Edit post clarification of the question:

Liquid nitrogen is still fun if you can find an excuse, and highly effective.

In a less drastic direction, simply doing the processing on pre-cooled (below 38F, preferably 33F) vegetables (and preferably in a walk-in cooler held below 38F) should keep things out of the botulism growth zone, and putting them in a -20F freezer for long enough (do some test runs where you actually stick a thermometer in a test bag) to get the center of the mass down below 0F before transfer to a 0F freezer (assuming that's your standard freezer temperature) would likely suffice. A cold brine step might help freezing get started faster but would be messy as compared to just using a -20F freezer (perhaps with racks to separate bags and extra air circulation, so you don't have bags packed together until after they are frozen through.) Depending on scale, maybe just size the -20F unit for a days production and clear it out at the start of each day after holding product overnight.

If processing in a warmer environment, you'll want to minimize time between "being held pre-cooled" and "being packed and frozen" - and especially "time between being vacuum-packed and being frozen." But giving the workers warm coats and hot coffee (and breaks) while working in a cooler will be safest. You will also need to sanitize any equipment on a regular schedule, which will be more often if they are not cold (and you'll need to check the temperature of the food contact parts of any machines when they have been running for a while.)