Mayonnaise and Vinaigrette Are Both Emulsions
An emulsion is oil and water forced to mix, which they normally resist doing — mayonnaise stays combined because egg yolk contains lecithin, an emulsifier that coats tiny oil droplets and keeps them suspended in the water-based portion indefinitely. A basic vinaigrette has no true emulsifier, so shaking or whisking it together only temporarily suspends the oil in droplets before they merge back and separate; adding a bit of mustard (which also contains natural emulsifiers) makes a vinaigrette hold together noticeably longer.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.