Question 1.
Well, working in advertising business for 6 years, I should say stock photography is no news at all for this industry. All designers have their CD collections or stock site accounts, otherwise they could not produce anything of value. So I think Stock photography and microstock vectors are simply co-exist side by side with the advertising industry. Advertising is a much broader and bigger market and it feeds microstocks.
Question 2.
a) In terms of graphic design - microstocks are the skeleton in designer's closet. Microstock give designers very good, (sometimes genius) vectors in seconds for a such a small price! Well it helps and it makes them lazy.
b) I've no idea about photo stocks influence on photography as an art, actually. Art photography is sold via posters, exhibitions, books, sometimes via stock agencies why not? Stock photography is sold solely via internet stock agencies. There is a proverb about that - "an amateur thinks of art, a professional thinks of money"
May be the best microstock masters are doing fine art when they work for stocks, but, in my opinion, these two fields (art and stocks) are too different, and serve for too different needs of society.