It is pretty common to use arbitrary string values in Option 66 to provision IP Phones. However, VyOS does not accept this for an unknown reason (which I consider a bug):
set service dhcp-server shared-network-name <network-name> subnet <subnet> static-mapping <mapping-name> option tftp-server-name "http://1.2.3.4:65000/provisioning/abcxyzdefghi/cfg{mac}"
will fail with:
Error: http://1.2.3.4:65000/provisioning/abcxyzdefghi/cfg{mac} is not a valid IPv4 address Invalid value Value validation failed Set failed
This should be an easy fix and perfectly backwards compatible.