If you don't need load balancing or failover, you may want to look at the VPN Policy Routing package to achieve what you want, rather than mwan3. To achieve what you want generally, it's policy based routing, which both packages can do.
mwan3 is however primarily designed for multiple independent WAN links and being able to configure load balancing or failover policies. VPN policy routing is primarily designed to allow you to route traffic to either VPN or WAN, depending on certain criteria such as IP, domain name, subnet (which is probably the best method for your VLAN case) and might be overall simpler for your use case. mwan3 can achieve what you want too, but the configuration required is probably more overkill for your requirements.
Mullvad does have it's own guide for configuring a Wireguard client: https://mullvad.net/en/help/running-wireguard-router/
Each Wireguard interface should be uniquely named so "wireguard" "wireguardb" etc. VPN policy routing will automatically pick up interfaces named as wireguard*.
Note: Mullvad IPv6 uses NAT6, you will also have to deploy to NAT6 to have working IPv6 policies as well.
If you are configuring multiple Wireguard clients through Mullvad, make sure to select servers in a different region and if possible using a different provider across each chosen endpoint in the event of any issues for redundancy.
Mullvad's server page will allow you to find all the available Wireguard endpoints under a region and their provider details.