The 877 allows for support of upto 2 vlans with the base image and one of them must be the DMZ.
It also allows for upto 10 Wireless vlans.
You can have two vlans but they cannot both point to the WAN IP as a default gateway.
A default gateway must be in the same subnet as the host attempting to use it.
ie: vlan1 host - 10.1.1.25/24; vlan2 host - 10.1.2.30/24
(the default gateway for a vlan1 host must be on the 10.1.1.0/24 subnet while a default gateway for a vlan2 host must be on the 10.1.2.0/24 subnet)
Please see the following links for more info on 87x series VLANs and IP Routing:
87x vlans config - http://www.cisco.com/en/US/docs/routers/access/800/850/software/configuration/guide/857sg_bk.pdf
ip routing q&a - http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012d8f7.shtml#qa15