Why don't we use-will you get some vegetables instead of - Would you get some vegetables

first of all "would" is the past tense of "will" . second "would" show uncertainty of something and "will" show more certainty of something. third "would" is more polite than "will". take a look upon those sentences.. would you open the door(friend or acquaintances) will you open the door(juniors or Youngers)