Reproducible from both CLI and API:
DBGvpp# create vhost-user socket /tmp/sock0
VirtualEthernet0/0/0
DBGvpp# set interface state VirtualEthernet0/0/0 up
DBGvpp# delete vhost VirtualEthernet0/0/0
0: /home/vpp/build-data/../vnet/vnet/devices/virtio/vhost-user.c:2263 (vhost_user_interface_admin_up_down) assertion `! pool_is_free (vum->vhost_user_interfaces, _e)' fails
Failed to save post-mortem API trace to /tmp/api_post_mortem.97462
Aborted (core dumped)