Is there any advantage of using VLAN over guest wifi in this situation
It depends on exactly how the guest WiFi is implemented, but usually it is done by creating another internal network which is strictly separated from the "normal" internal network. Sometimes are also additional restrictions possible for the guest network, like separation between clients inside this guest network (i.e. cannot see and connect to each other) and restricted internet access (like only HTTP and HTTPS).
A VLAN has no advantage here regarding separation, it might even provide less restrictions than offered with the guest network (i.e. no separation of clients inside the VLAN, no restricted internet access).
not be held liable for their internet activity
This can not be achieved, neither with a guest network nor with a VLAN. Both share the same external IP address. It is not clear from outside if some access to the internet originated in the guest network or normal network.
There is a business model though to repurpose home internet routers as public WiFi hotspots. In this case the provider of this business model will take care of the liability problems. This is usually done by routing the hotspot traffic using a VPN to a VPN exit of the hotspot provider. This makes the hotspot provider the public visible source IP of the traffic and thus any legal problems are directed to the hotspot provider. Sometimes this kind of feature is already available on the routers provided by the local ISP and sometimes it even already enabled. See for example Your Home Router May Also Be a Public Hotspot — Don’t Panic!. Note that access to this hotspot is then usually open to all public, but often restricted to anybody having an agreement with the hotspot provider which might come with additional costs.
