I had an Alix 2D13 running 2 WANs on the physical WAN port, by VLANs, 3 VLANs on the OPT1 port for various local subnets and the real LAN port being an ordinary LAN (I did that just so that I can always easily get to the webGUI by connecting physically to LAN, even if all the VLAN switch configs are gone).
No trouble running VLANs on that.
With multiple WANs I have trouble during failover - with 2.1.* at lot of stuff fires up at once failing over OpenVPN server/clients, doing DynDNS updates… and it sometimes runs out of the 256MB in real time. That means some process/es implementing the failover changes get killed, and so some things in the failover do not always implement. I believe this should be better in 2.2. Anyway, just saying that in a multi-WAN scenario on 2.1.* 256MB is not quite enough memory.