Comment in line Andreas John wrote:
4.) Hint: If you setup VLAN with /etc/network/interfaces please keep in mind that the physical interface has to be up in order to create VLANs on it, i.e. you have to set somme (dummy?) IP to ethX in order to create ethX.VID.
You'd better leave up but unconfigured the physical interface in order to avoid further routing trouble.
HTH, Andreas
-- j