Within the vSphere UI, you can view a detailed breakdown of your vSAN storage utilization including various system overhead by selecting a specific vSAN Cluster and then navigating to Monitor->vSAN->Capacity as shown in the screenshot below.
Different vSAN configuration such using vSAN Original Storage Architecture (OSA) or vSAN Express Storage Architecture (ESA) and whether capabilities like vSAN Deduplication and Compression is enabled will yield different utilization metrics that are displayed.
I recently had an inquiry asking how to retrieve the vSAN Deduplication and Compression overhead information using PowerCLI?