Symptoms:
vCenter is offline due to e.g.:
- vCenter Server ( VM ) is currently being upgraded
- vCenter Server ( VM ) is inaccessible
- vCenter Server ( VM ) is broken
- vCenter Server ( VM ) is being rebooted

VMware vSAN 6.x

There is no impact to vSAN if vCenter is offline or not fully functional in any way.
vSAN will continue to operate normally and Virtual Machines (VMs) will continue to run. 
vSAN operates independently from vCenter.
vCenter (= Web Client ) only serves as Management & Monitoring tool for vSAN.

If vCenter (Web Client) is not available, the status of vSAN can be checked by connecting to one of the vSAN Hosts via SSH/Putty:

1.) Verify that all vSAN Hosts are in the Cluster:

Example ( output may vary a little based on versions ) : 
The number should reflect the total number of vSAN Hosts which belong to this Cluster. 
If you see Sub-Cluster Member Count not reflecting the total number of vSAN Hosts that belong to this Cluster, please open a case with VMware to have this further investigated.

[root@esx1:~] localcli vsan cluster get
Cluster Information:
   Enabled: true
   Current Local Time: 2020-05-15T09:34:12Z
   Local Node UUID: 5cf75ee4-271c-b3d1-18d1-00505601607f
   Local Node Type: NORMAL
   Local Node State: AGENT
   Local Node Health State: HEALTHY
   Sub-Cluster Master UUID: 5cf72141-e477-d73c-51cc-00505601e3c0
   Sub-Cluster Backup UUID: 5cf7213e-3249-0e1c-dc02-00505601e3c8
   Sub-Cluster UUID: 523225cb-243c-7baa-1c0e-a07ee7b14900
   Sub-Cluster Membership Entry Revision: 5
   Sub-Cluster Member Count: 7  *************** >  The number should reflect the total number of vSAN Hosts which belong to this Cluster 
   Sub-Cluster Member UUIDs: 5cf7213e-3249-0e1c-dc02-00505601e3c8, 5cf71fe4-ca67-7232-08da-00505601608f, 5cf72141-e477-d73c-51cc-00505601e3c0, 5cf7a659-fde5-e5a0-bae9-00505601e5dc, 5cf75ee4-271c-b3d1-18d1-00505601607f, 5cf71fa3-e09a-3779-4b5f-005056016087, 5cf72145-6fc5-4a23-1fa0-00505601e3d1
   Sub-Cluster Membership UUID: f825b25e-3917-6291-df8a-00505601e3c0
   Unicast Mode Enabled: true
   Maintenance Mode State: OFF
   Config Generation: fd9b3efb-2c1f-43f4-a691-1e7fc89ebcc5 9 2020-05-08T04:43:26.262

2.) Check the Health of all vSAN Objects located on the Cluster
This refers to the Health Status of your data located on the vSAN Datastore.

Option A:
Example: below shows all vSAN Objects (= 40) in the Cluster are healthy.
Additional Infos in regards to listed states in the output below

[root@esx1:~] localcli vsan debug object health summary get
Health Status                                     Number Of Objects
-------------------------------------------------------------------
inaccessible                                                      0
reduced-availability-with-active-rebuild                          0
reduced-availability-with-no-rebuild                              0
data-move                                                         0
nonavailability-related-incompliance                              0
healthy                                                          40
reduced-availability-with-no-rebuild-delay-timer                  0
nonavailability-related-reconfig                                  0

Option B:
state\": 7 ---> "Healthy"
If you see anything else apart from "state\": 7", please open a case with VMware to have this further investigated. 

[root@esx1:~] cmmds-tool find -f python | grep CONFIG_STATUS -B 4 -A 6 | grep 'uuid\|content' | grep -o 'state\\\":\ [0-9]*' | sort | uniq -c
     40 state\": 7   *************** > Showing that 40 Objects are healthy


3.) Run a shortened Version of the vSAN Healthcheck
Example: below is showing that all Health Test reporting a healthy status ( green )
If you see in the output below any Health Test listed with status "yellow" or "red", 
please open a case with VMware to have this further investigated.

[root@esx1:~] localcli vsan health cluster list

Health Test Name                                    Status
--------------------------------------------------  ------
Overall health                                      green
Network                                             green
  Hosts with connectivity issues                    green
  vSAN cluster partition                            green
  All hosts have a vSAN vmknic configured           green
  vSAN: Basic (unicast) connectivity check          green
  vSAN: MTU check (ping with large packet size)     green
  vMotion: Basic (unicast) connectivity check       green
  vMotion: MTU check (ping with large packet size)  green
  Network latency check                             green
Data                                                green
  vSAN object health                                green
  vSAN object format health                         green
Cluster                                             green
  Advanced vSAN configuration in sync               green
  vSAN daemon liveness                              green
  vSAN Disk Balance                                 green
  Resync operations throttling                      green
  Software version compatibility                    green
  Disk format version                               green
Physical disk                                       green
  Operation health                                  green
  Disk capacity                                     green
  Congestion                                        green
  Component limit health                            green
  Component metadata health                         green
  Memory pools (heaps)                              green
  Memory pools (slabs)                              green
Capacity utilization                                green
  Storage space                                     green
  Read cache reservations                           green
  Component                                         green
  What if the most consumed host fails              green
Performance service                                 green
  Stats DB object                                   green
  Stats primary election                            green
  Performance data collection                       green
  All hosts contributing stats                      green
  Stats DB object conflicts                         green
[root@esx1:~] 

• A blog from VMware mentioned below has given the detailed information to user about it.