Thanks for reaching out.
Unfortunately, you cannot use Azure Key Vault for Azure AD B2C Application.
Accessing Key Vault is not supported with token obtained using B2C user flow or custom policy. You can request a token using same way that you used with standard Micrsoft Entra ID i.e. using login.microsoftonline.com for B2C tenant as well and access Key Vault using that token.
Hope this will help.
Thanks,
Shweta
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