Declining Employee Loyalty: A Casualty of the New Workplace
knowledge.wharton.upenn.edu
knowledge.wharton.upenn.edu
I've found vesting periods to be prohibitive enough that I don't even consider these things when negotiating employment.
Also, I'd add that job-hopping (what I'd define as working somewhere less than 12-24 months) seems to have been de-stigmatized – at least in larger tech hubs.