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No, you cannot take snapshots at all - this is a caveat for WSFC in general and is not specific to vSAN:
Yes, you can use agent-based backup I side the Guest OS - Veeam have some guides with further information on this if you have a search.
So If I understand you well. I should treat the WSFC on vSAN as an physical failover cluster from backup perspective. I'm familair with backing up physical Windows failover clusters. This sounds as an interesting test for in my lab.
Thank you for the quick response.