Question: When did the term "vegetable" become synonymous with plants grown for food?
About the article "Vegetable", section "Etymology".

Candidate passages:
The word "vegetable" was first recorded in English in the early 15th century from Old French, and was in origin applied to any plant.
This is still the sense of the adjective "vegetable" in biological context.
The word is taken from Medieval Latin "vegetabilis" "growing, flourishing" (i.e.
of a plant), a semantic change from a Late Latin meaning "to be enlivening, quickening", a derivation of the verb "vegetare" "enliven", which is derived from "vegetus" "to be alive, active" (related to "vigor"), in reference to the process of a plant growing.
In 1767, the meaning of the term "vegetable" was specified to mean "plant cultivated for food, edible herb or root."
The year 1955 noted the first use of the shortened, slang term "veggie".
As an adjective, the word "vegetable" is used in scientific and technical contexts with a different and much broader meaning, namely of "related to plants" in general, edible or not — as in "vegetable matter", "vegetable kingdom", "vegetable origin", etc.
The meaning of "vegetable" as "plant grown for food" was not established until the 18th century.
A vegetable garden (also known as a vegetable patch or vegetable plot) is a garden that exists to grow vegetables and other plants useful for human consumption, in contrast to a flower garden that exists for aesthetic purposes.
It is a small-scale form of vegetable growing.
A vegetable garden typically includes a compost heap, and several plots or divided areas of land, intended to grow one or two types of plant in each plot.
Plots may also be divided into rows with an assortment of vegetables grown in the different rows.
It is usually located to the rear of a property in the back garden or back yard.
Many families have home kitchen and vegetable gardens that they use to produce food.
In World War II, many people had a garden called a "victory garden" which provided food and thus freed resources for the war effort.
With worsening economic conditions and increased interest in organic and sustainable living, many people are turning to vegetable gardening as a supplement to their family's diet.
Food grown in the back yard consumes little if any fuel for shipping or maintenance, and the grower can be sure of what exactly was used to grow it.
Organic horticulture, or organic gardening, has become increasingly popular for the modern home gardener.
There are many types of vegetable gardens.
The potager, a garden in which vegetables, herbs and flowers are grown together, has become more popular than the more traditional rows or blocks. 
Several species, notably in the genus "Sagittaria", have edible rhizomes, grown for both human food and animal fodder in southern and eastern Asia.
They were eaten as food by the indigenous peoples of North America.
Most have value as food for wildlife.
Some are grown as ornamental plants in bog gardens, ponds and aquariums.
"Limnocharis flava"'s leaves and flower buds are eaten in Southeast Asia as "poor people's vegetable". 
One possible environmental factor in the food price crisis is rising background levels of ground-level tropospheric ozone in the atmosphere.
Plants have been shown to have a high sensitivity to ozone levels, and lower yields of important food crops, such as wheat and soybeans, may have been a result of elevated ozone levels.
Ozone levels in the Yangtze Delta were studied for their effect on oilseed rape, a member of the cabbage family that produces one-third of the vegetable oil used in China.
Plants grown in chambers that controlled ozone levels exhibited a 10–20 percent reduction in size and weight (biomass) when exposed to elevated ozone levels.
Production of seeds and oil was also reduced.
The Chinese authors of this study also reported that rice grown in chambers that controlled ozone levels exhibited a 14 to 20 percent reduction in biomass yield when exposed to ozone levels over 25 times higher than was normal for the region. 
The bulb is edible as a root vegetable, cooked or dried, and can be ground into flour.
The leaves can also be cooked as a leaf vegetable.
In Japan, "Erythronium japonicum" is called "katakuri", and the bulb is processed to produce starch, which is used for food and other purposes.
They are also widely grown as ornamental plants, with numerous hybrids and cultivars having been selected for garden use.
Popular cultivars include "Erythronium" 'Pagoda', "E." 'Sundisc', "E." 'Joanna', "E." 'Kondo', "E." 'Citronella', "E. californicum" 'White Beauty', and "E." 'Rosalind'.
Propagation is best by seed in autumn or by division of bulbs, depending on species.
Some species propagate vegetatively.
The plant is also great as a ground cover, as it will spread over several years.