Addon does not survive during system updates and must be reinstalled for each new VyOS version.
Some types of traffic (like multicast) may not work on VPP-enabled interfaces. Currently tested only unicast. Feel free to provide your feedback, if you will find any traffic type that does not work.
Features like firewall or PBR which use Linux kernel are not exposed to VPP dataplane. Traffic that has both in and out interfaces inside VPP cannot be controlled additionally.
vmxnet3 interfaces require exposing/emulation IOMMU for a VyOS virtual machine.
virtio interfaces are not supported with IOMMU enabled on a VyOS virtual machine
routes that are not available in the kernel will not be added to VPP, including the default route. All traffic to non-existing routes is dropped. This means that to receive all the traffic it is better to have a default route in the kernel - then VPP will punt everything not routable via VPP interfaces to the kernel. An example of the corner case is a DHCP client that will not work because DHCP answers will not be forwarded to the kernel without the 0.0.0.0/0 route.