Got this all working. Since there wasn't a straight forward way for an interface link to be up, but without an IP address for VM bridging purposes, I decided to statically configure RFC 3927 addresses for those two interfaces, ensuring the link would be up. The goal was to allow only the VyOS VM to DHCP from the cable modem to get the public IP address.
My XPEnology has 3 ethernet interfaces...
eth0 - Static 10.0.1.6/24 - LAN
eth1 - Static 169.254.169.254/16 - Cable Modem
eth2 - Static 169.254.169.253/16 - LAN
The VyOS VM has 2 ethernet interfaces...
eth0 - DHCP - Cable Modem (bridged via host's eth1)
eth1 - Static 10.0.1.1/24 - LAN (bridged via host's eth2)
VyOS handles all DHCP, NAT, and Firewall services for the LAN. This setup also allows other VMs to bridge via host's eth2, and to get an IP address on the LAN.
Performance is great. Iperf shows near gigabit speeds to the VyOS VM. Internet connection completely saturated (300/20 mbit + 10% overprovision).
vyos@vyos:~$ iperf -B 10.0.1.1 -s
------------------------------------------------------------
Server listening on TCP port 5001
Binding to local address 10.0.1.1
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[ 4] local 10.0.1.1 port 5001 connected with 10.0.1.5 port 64755
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 1.09 GBytes 937 Mbits/sec
Mac-mini:~ bgmccollum$ iperf -c 10.0.1.1
------------------------------------------------------------
Client connecting to 10.0.1.1, TCP port 5001
TCP window size: 129 KByte (default)
------------------------------------------------------------
[ 4] local 10.0.1.5 port 64755 connected with 10.0.1.1 port 5001
[ ID] Interval Transfer Bandwidth
[ 4] 0.0-10.0 sec 1.09 GBytes 938 Mbits/sec
Mac-mini:~ bgmccollum$ speedtest-cli --server 6249
Retrieving speedtest.net configuration...
Retrieving speedtest.net server list...
Testing from Time Warner Cable (x.x.x.x)...
Hosted by Vivint Wireless (San Antonio, TX) [26.90 km]: 33.408 ms
Testing download speed........................................
Download: 334.73 Mbit/s
Testing upload speed..................................................
Upload: 22.50 Mbit/s