Hi,
TLDR: A live vSAN environment with a total connectivity loss for an extended period of time will almost certainly result in problems. Putting Datastores in Maintenance mode will take it completely out of service and stop all I/O on it.
vSAN handles connectivity loss according to this documentation: Network Connectivity Is Lost in the vSAN Cluster (vmware.com)
You will have all Hosts isolated which will most likely result in I/O errors for the VMs. This will most likely lead to reboots of the VMs and could cause some data inconsistency on the OS level.
The best way to resolve a situation like yours would be to replace the network infrastructure step-by-step in parts, so keeping parts of the connectivity alive. If you have over 800VMs which are of such importance, why is there no network redundancy present in your deployment?