Socio-Economic Consequences of Nation-Wide Vegan Diet An advice I often hear regarding our environmental impact is to 'stop consuming meat and dairy products'. This is stated as the number one way to reduce your ecological footprint, according to multiple sources. Whether this is true or not, obviously much of our economy and society depends on the meat and dairy industry, ranging from farming and transport to butcheries or supermarkets.

If everyone in the United States would stop consuming meat, dairy and eggs within the next twelve months, what would be the economic consequences? 
How much of a hit would the economy take? 
What percentage of farmers could realistically replace their current products with vegan alternatives?
What other unintended socio-economic consequences would this scenario have?
I think your questions are very interesting. I am no expert on US farming data sources. However, I have briefly pursued these questions for the UK. As currently this question has no responses, I'll post my findings below verbatim along with my data sources.

I encourage you to pursue these research questions further, perhaps using a similar framework to below. 

"My response uses DEFRA data to examine the extent of the agri-food economy that must change, in equilibrium, to complete a transition to a Vegan Economy. Although in the short run Rural Unemployment may rise, once we distribute the gains from a Vegan Economy, we can raise rural incomes and reduce rural unemployment.

Output Data

DEFRA's Food Statistics in your Pocket reveals that the Agri-Food Industry made up 6.3% of Gross Value Added in 2016. However, a Vegan Economy only entails removing animal products, which in equilibrium is unlikely to influence the distribution, retail, or catering sectors negatively; people under the vegan norm must consume sufficient calories to be healthy. 

Note: The Vegan Norm assumes that people spend, counterfactually, the same proportion of income and that they eat a healthy, varied Vegan Diet. We also assume that UK Welfare can distribute any economic gains from a Vegan Economy. 

Only Agriculture, Fishing, and Food Manufacturing remain relevant, which has ~2% GVA contribution. This figure overestimates the contribution of Animal Products as plant-based firms can partially fill in for Food Manufacturing. Also, these figures fail to distinguish between animal and non-animal products. The ratio of the value of plant to animal products for imports or exports is 2 to 1, calculated from Observation of Economic Complexity (OEC) data, which seems a reasonable prior for domestic production too. This data also indicates that the UK balance of trade does not suffer.

Yet, the Vegan Economy also sees growth in GVA from old and new Vegan Alternatives to Animal Products. The market for alternative products is growing rapidly across the agri-food chain. We reference The Vegan Society who have collated such evidence on their website. As such, we can expect this market to fill in the anticipated output gap.  

Thus, in equilibrium, the Vegan Economy may see at most a 1% reduction in Gross Value Added from eliminating Animal Products from Agriculture and Food Manufacturing. Trend GVA growth may even rise thanks to alternative products.

Employment Data

DEFRA's Pocketbook indicates that we employ 820,000 people in Agriculture, Fishing, and Food Manufacturing. Animal Products are more labour intensive but likely make up only a third of GVA. So, perhaps half of these workers (~1% of UK Labour Force) have their employment at risk under a Vegan Economy. 

A sectoral shift towards crops and alternatives could fill in the output gap, suggesting that the general equilibrium effects on employment are likely negligible.

Yet, even for those who do lose their work, we can redistribute the benefits of a Vegan Economy. These benefits include increased government revenue from higher GVA growth, reduced healthcare costs and improved productivity from healthier workers. We also avoid costs associated with accelerated climate change.

Our options for helping those farmers or manufacturers out of work include but are not limited to:
incentives to switch to crop farming
start-up programs to encourage new land use such as afforestation or sanctuaries
incentives for transport, retail, accommodation, or local government to provide training or apprenticeship schemes for those out of work. They are often the main employers in rural areas."

UK Economic Data Sources:

DEFRA (October 2018), Food Statistics in your pocket 2017 Summary
url: https://www.gov.uk/government/publications/food-statistics-pocketbook-2017/food-statistics-in-your-pocket-2017-summary

Page 7 of Rural productivity and Gross Value Added (GVA) statistics, DEFRA (2019)
url: https://www.gov.uk/government/statistics/rural-productivity

Observation for Economic Complexity (2017), United Kingdom
url: https://oec.world/en/profile/country/gbr/

The Vegan Society (2019), The Vegan Society Statistics: Business/Food Sales
url: https://www.vegansociety.com/news/media/statistics

Public Health Evidence

Chen, J., Peto, R. Pan, W., Liu, B., Campbell, T.C. Mortality, Biochemistry, Diet and Lifestyle in Rural China. Geographic study of 69 counties in Mainland China and 16 areas in Taiwan. Oxford University Press, 2006
url: https://nutritionstudies.org/the-china-study/

Academy of Nutrition and Dietetics (2016), The Position of the Academy of Nutrition and Dietetics: Vegetarian Diets
url: https://www.eatrightpro.org/-/media/eatrightpro-files/practice/position-and-practice-papers/position-papers/vegetarian-diet.pdf

British Dietetic Association (August 2017), British Dietetic Association confirms well-planned vegan diets can support healthy living in people of all ages 
url: https://www.bda.uk.com/news/view?id=179

Environmental Evidence

Poore and Nemecek (2018), Reducing food’s environmental impacts through producers and consumers

CH5 page 77 Figure 5.12 of 
Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems (IPCC 2019)
url: https://www.ipcc.ch/srccl-report-download-page/