Hello @DMC ,
You can create Vnet peering between both the virtual networks existing in different subscriptions (which may belong to same/different Azure Active Directory (Azure AD) tenants), provided the virtual networks you peer must have non-overlapping IP address spaces.
Please refer : https://zcusa.951200.xyz/en-us/azure/virtual-network/create-peering-different-subscriptions
To know more about the requirements and constraints of Vnet peering, please refer : https://zcusa.951200.xyz/en-us/azure/virtual-network/virtual-network-manage-peering#requirements-and-constraints
If you would like to access both the Azure Vnets (deployed in different subscriptions) from your On-premises via the Site to site VPN post Vnet peering configuration, you can do so by configuring gateway transit for virtual network peering. Gateway transit is a peering property that enables one virtual network to utilize the VPN gateway in the peered virtual network for cross-premises or VNet-to-VNet connectivity.
For more information, please refer : https://zcusa.951200.xyz/en-us/azure/vpn-gateway/vpn-gateway-peering-gateway-transit
Kindly let us know if the above helps or you need further assistance on this issue.
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