While this is true for some people, the number affected is likely to be low.

 - Polling places are [generally open for at least 12 hours][1], so being unable to vote due to work would mean you would need a very long commute or be working very long hours.

 - Depending on your state and your circumstances you should be able to vote early or by post if you can't get to your local polling place on election day. However this requires planning ahead and the details vary across the country, so some eligible voters might not [get through the bureaucracy][2]. 

For those who find they need to take time off it depends on the employer. Most employers will allow a vacation day given enough notice. This will be paid, but come out of the annual vacation allowance. Some employers will allow half days, others don't. Some may simply refuse to allow that particular day to be taken off on the grounds of staff shortage. Simply not turning up is a violation of your employment contract, not an "unpaid vacation day".

Of course US elections are so finely balanced that even a small number of voters can [make a difference][3].


  [1]: https://ballotpedia.org/State_Poll_Opening_and_Closing_Times_(2016)
  [2]: https://en.wikipedia.org/wiki/Voter_suppression_in_the_United_States#Limitations_on_early_and_absentee_voting
  [3]: https://en.wikipedia.org/wiki/2000_United_States_presidential_election_recount_in_Florida