Comment 8 for bug 1295304

The problem specifically here is that both vlan's post-down hook and ifupdown both try to destroy the vlan, which obviously fails.
We could obviously make the deletion of the interface in ifupdown non-fatal, but I'm not convinced it's the right course of action if you intend people to use ifupdown directly for vlan management.

As for what seems to be lacking in ifupdown's own vlan implementation, from a quick look at the code, it looked like the following:
 - No way to trigger the vlan device creation when the parent device appears (from udev hook) which is required for event based network bring up in Ubuntu
 - Doesn't support all the interface name syntaxes supported by vlan's ifupdown hooks

For bridges, only the first of those two applies (we need a way for udev to create any relevant bridge when a slave first appears on the machine).