The cost of holding apartments intentionally vacant is large, and has a very tiny effect on pricing unless there's a single player with more market share than exists anywhere I have seen in the US.
In a different arena, parking, it's widely accepted that the best possible way to establish adequate parking is to set prices high enough that there's always a spot available. Anything less results in people not being able to park. Setting apartment prices similarly is beneficial to the public, in that there's always a place to move into.
We need apartment vacancies, they are beneficial to renters, because people need to find housing at all times. Ideally we have a surplus of apartments so that not only do renters have choices but so that prices fall to the cost of providing housing.