vhost_user_starter is a daemon that serves dbus request from concierge to create new vhost-user device for concierge. See /vm_tools/README.md for details.
If you are modifying protocols, cros_workon chromeos-base/system_api dev-rust/system_api.
cros_workon --board ${BOARD} start \
dev-rust/system_api chromeos-base/vhost_user_starter
Then it is possible to work on vhost_user_starter.
emerge-${BOARD} dev-rust/system_api chromeos-base/vhost_user_starter
If you want to build vhost_user_starter by using cargo build, you need enter chroot:
cros_sdk (chroot) cd /mnt/host/source/src/platform2/vm_tools/vhost_user_starter && cargo build