Alexey, this indeed has nothing to do with NetworkManager.
If you just do:
sudo ifdown eth0.20
sudo ifup eth0.20
Is the device properly initialized and brought up?
** Package changed: network-manager (Ubuntu) => vlan (Ubuntu)
** Changed in: vlan (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/717243
Title:
vlan interface won't start with ifup, can be started manually
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