It seems impossible to build helium or lithium ISO images anymore. Both branches now use a version of vyatta-quagga that depends on eglibc 2.15, which of course is jessie, and not in any way available on squeeze.
From Packages:
Package: vyatta-quagga
Version: 0.99.20.1-14+vyos2+lithium9
Architecture: amd64
Maintainer: VyOS Package Maintainers <maintainers@vyos.net>
Installed-Size: 6484
Pre-Depends: adduser
Depends: libc6 (>= 2.15), libcap2 (>= 1:2.10), libpam0g (>= 0.99.7.1), libreadline6 (>= 6.0), libsnmp30 (>= 5.7.2.1+dfsg-1~dfsg), libssl1.0.0 (>= 1.0.0), libtinfo5, logrotate (>= 3.2-11), iproute
Excerpt from installation log:
The following packages have unmet dependencies:
vyatta-quagga: Depends: libc6 (>= 2.15) but 2.11.3-4+deb6u4 is installed.
Depends: libsnmp30 (>= 5.7.2.1+dfsg-1~dfsg) which is a virtual package.
Depends: libssl1.0.0 (>= 1.0.0) which is a virtual package.
Depends: libtinfo5 which is a virtual package.
The following actions will resolve these dependencies:
Keep the following packages at their current version:
1) vyatta-cfg-dhcp-server [Not Installed]
2) vyatta-cfg-op-pppoe [Not Installed]
3) vyatta-cfg-quagga [Not Installed]
4) vyatta-cluster [Not Installed]
5) vyatta-config-mgmt [Not Installed]
6) vyatta-conntrack [Not Installed]
7) vyatta-cron [Not Installed]
8) vyatta-dhcp3-server [Not Installed]
9) vyatta-nat [Not Installed]
10) vyatta-netflow [Not Installed]
11) vyatta-op [Not Installed]
12) vyatta-op-dhcp-server [Not Installed]
13) vyatta-op-firewall [Not Installed]
14) vyatta-op-qos [Not Installed]
15) vyatta-op-quagga [Not Installed]
16) vyatta-op-vpn [Not Installed]
17) vyatta-openvpn [Not Installed]
18) vyatta-quagga [Not Installed]
19) vyatta-ravpn [Not Installed]
20) vyatta-vrrp [Not Installed]
21) vyatta-wanloadbalance [Not Installed]
22) vyatta-webgui [Not Installed]
23) vyatta-webproxy [Not Installed]
24) vyatta-wireless [Not Installed]
25) vyatta-wirelessmodem [Not Installed]
26) vyos-nhrp [Not Installed]
27) vyos-pppoe-server [Not Installed]