Hi Bro
Just assign VLAN 1234 to the admin context. As for the other contexts, ensure those vlans assigned to those context are routable to VLAN 1234, and are you're able to poll SNMP, SYSLOG etc. messages.
Yes, you can assign VLAN 1234 to multiple context, but this is not practical. This is because each contect will require 2 IP Address for VLAN 1234, since it's running in failover mode. If you were to run 250 contexts (maximum), this approach will not work. You'll run out of VLAN 1234 IP Address in no time.
This is surely not the best design. You could refer to http://www.ciscopress.com/articles/article.asp?p=426641 for more details on this.
P/S: If you think this comment is useful, please do rate them nicely :-)
Warm regards,
Ramraj Sivagnanam Sivajanam