After scouring the internet for answers on howto use Busybox’s ifupdown binary and configure /etc/network/interfaces to set a vlan for an interface I looked in the ifupdown.c code and there are no references to vlans at all!
Don’t panic! Just because there isn’t built in support for setting the vlans doesn’t mean you can’t use /etc/network/interfaces or have to resort to using some custom cockamamie script to set your vlans up! The answer is in front of your nose!
Lately I’ve been making extensive use of the ifupdown pre-up and post-up options for doing insane things like appending iptables rules, bringing up and down other interfaces, etc… The simple trick is to use pre-up|up to use vconfig to create your vlan. The configuration file’s default location is /etc/network/interfaces