TD;DR
I found Synology DSM 7.2.2 supports internal(private) vSwitch with IPv6 support ONLY (without using a physical switch). No IPv4! (I assume Synology won't like users to use internal(private) vSwitch because it costs lots of CPU)
add the same private switch to debian vm A (as server) and debian vm B (as client)
Add this to /etc/network/interfaces to server A and client B. ens4 is the virtual private switch on DSM.
# The 2nd network interface
allow-hotplug ens4
iface ens4 inet dhcp
restart the network
systemctl restart networking.service
ip addr
Output
3: ens4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 02:11:32:2e:aa:02 brd ff:ff:ff:ff:ff:ff
altname enp0s4
inet6 fe80::11:32ff:fe2e:aa02/64 scope link
valid_lft forever preferred_lft forever
On server A (it's ethernet interface is ens4 and IPv6 is fe80::11:32ff:fe24:9f2e)
iperf3 -s --bind fe80::11:32ff:fe24:9f2e%ens4
Above we bind specific IPv6(server side) on specific interface(server side)
On client B (it's ethernet interface is ens4 and IPv6 is fe80::11:32ff:fe2e:aa02)
iperf3 --client --bind fe80::11:32ff:fe2e:aa02%ens4 --client fe80::11:32ff:fe24:9f2e%ens4
Above we bind specific IPv6(client side) on specific interface(client side) to reach specific IPv6(server side) on specific interface(client side)
You will get this on client B
Connecting to host fe80::11:32ff:fe24:9f2e%ens4, port 5201
[ 5] local fe80::11:32ff:fe2e:aa02 port 41314 connected to fe80::11:32ff:fe24:9f2e port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 962 MBytes 8.06 Gbits/sec 0 3.02 MBytes
[ 5] 1.00-2.00 sec 826 MBytes 6.94 Gbits/sec 1 3.02 MBytes
[ 5] 2.00-3.00 sec 910 MBytes 7.63 Gbits/sec 0 3.02 MBytes
[ 5] 3.00-4.00 sec 718 MBytes 6.01 Gbits/sec 0 3.02 MBytes
[ 5] 4.00-5.00 sec 769 MBytes 6.46 Gbits/sec 0 3.02 MBytes
[ 5] 5.00-6.00 sec 841 MBytes 7.05 Gbits/sec 0 3.02 MBytes
[ 5] 6.00-7.00 sec 909 MBytes 7.62 Gbits/sec 0 3.02 MBytes
[ 5] 7.00-8.00 sec 791 MBytes 6.64 Gbits/sec 0 3.02 MBytes
[ 5] 8.00-9.00 sec 871 MBytes 7.31 Gbits/sec 0 3.02 MBytes
[ 5] 9.00-10.00 sec 728 MBytes 6.10 Gbits/sec 1 3.02 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 8.13 GBytes 6.98 Gbits/sec 2 sender
[ 5] 0.00-10.00 sec 8.13 GBytes 6.98 Gbits/sec receiver
So VMs on the DSM virtual private switch (VirtIO) works on IPv6 (internal).
I haven't found a method to make it work on Internet IPv4 since it doesn't get a IPv4 address, such as 10.0.0.x or 172.0.x.x
Useful info about using iperf3 with IPv6 esnet/iperf#618 (comment)
Network info from Synology https://kb.synology.com/en-br/DSM/help/Virtualization/network?version=7
After I tested the performance, I found when starting iperf3 test on client vm and server vm, the host CPU (on NAS) usage reached to more than 90%, maybe that's why Synology won't add IPv4 vSwitch support, which means you can use internal vSwitch IPv6 but not recommanded.