If you have a single network interface and want to have your VM's in a VLAN, modify the /etc/network/interfaces to look something like:

auto lo
iface lo inet loopback

iface eno1 inet manual

auto vmbr0
iface vmbr0 inet static
	bridge-ports eno1
	bridge-stp off
	bridge-fd 0

auto vmbr1
iface vmbr1 inet manual
	bridge-ports eno1.20
	bridge-stp off
	bridge-fd 0

iface eno1.20 inet manual
	vlan-raw-device eno1

source /etc/network/interfaces.d/*

After modifying apply the changes:

systemctl restart networking

In this case, thee is only one interface eno1 and the VLAN-ID is 20