When people talk about vegetarianism using 'less energy' than being omnivorous or carnivorous, they are using numbers based on petroleum-intensive artificial agriculture: pesticide farming, long transports to market, petroleum feedstock-based fertilizers, feedlot grown beef, chickens crammed into long houses laying eggs onto conveyer belts. In reality, it's not quite that simple.

I believe that the various options are, from "most usage" to "least usage" are:

1) conventional, non-organic omnivorous diet from conventional retail
2) conventional, non-organic vegetarian diet from conventional retail
3) omnivorous diet from food co-op, farmer's market, local CSA
4) veggie/vegan diet ditto
5) veggie/vegan diet supplemented by sustainable backyard permaculture (veggies, fruits, worms for composting only)
6) omnivorous diet from sustainable backyard permaculture (veggies, 1 - 2 fruits, worms for compost + chickens, chickens for eggs and meat)
7) omnivorous diet from a sustainable quarter-acre+ permaculture supporting veggies, fruit, worms, chickens, and possibly a 'summer pig' raised on wurzel, beet, stand corn, etc plus garden scraps
8) veggie/vegan diet from sustainable quarter-acre+ permaculture

Why would I put 5 & 6 in their current order? Because on a generic backyard, the protein density one can achieve with eggs and chickens is much more than that which one can do with legumes of any kind. It's not clear that it is possible for a vegan individual or a small family to raise most of their own protein in a small backyard. Ovo-vegetarians may be able to do so.

Ovo-lacto vegetarians on 2+ acres can probably set up a garden/orchard/pasture rotation such that they can be mostly self-sufficient without vast mechanical overhead. Pure vegans are going to need mechanical assistance, including some kind of power (be it solar or wind supplied, still power), though it is technically possible. See Gene Logsden's book on Small-Scale Grain Farming.

So there's more to consider than just meat or veggie or vegan. A vegetarian who lives in a single-family home and buys a lot of fruit and veggies out of season, transported from other countries or raised in heated greenhouses, and wears only natural fibers may have a worse eco-footprint than an omnivore who lives in high-density housing near or in an urban core and who shops primarily at their local farmers market, buying fruit & veggies in season, and wears a lot of fleece (recycled soda bottles) or shops for clothes at thrift stores.

There are a lot of variables. I am trying to figure out where I could put a small chest freezer in our home. It would cost about $200 up front, and $28/year in electricity (according to the efficiency rating). I could buy one or more chicken "shares" from a pasture-raising farm where they only slaughter quarterly, and stop buying non-local, grain-fed chicken. I could also preserve items like local organic strawberries, my backyard tomatoes, etc more easily and efficiently by freezing them (in reusable containers) than I do by canning them, which takes more energy. Etc.

What's most important is that we all do what we can, and that we try to expand our own levels of doing a little bit more every few weeks. In a year, it makes a lot of difference.