Rack failure protection can be achieved by assigning nodes within the same rack to different Failure Domains.
Failure Domains: When using Failure Domains, one instance of the data is kept within the local Failure Domain and another instance of the data is kept within another Failure Domain. By applying Failure Domains, rack failure protection can be achieved as well as site failure protection in stretched configurations.
VMware, vSAN [SDS]/7.0 U1, Enterprise
Rack Failure Protection, Reads/Writes, Data Availability