HI
Post doing a format install you may do a device mode change.
You could also try removing the vc files from the system
% cd /config/
% ls
db ssh_host_dsa_key ssh_host_rsa_key
juniper.conf.1.gz ssh_host_dsa_key.pub ssh_host_rsa_key.pub
juniper.conf.2.gz ssh_host_ecdsa_key usage.db
juniper.conf.3.gz ssh_host_ecdsa_key.pub vchassis
juniper.conf.gz ssh_host_key
juniper.conf.md5 ssh_host_key.pub
% cd vchassis/
% ls
vc.db vc.tlv.db.0 vclocal.conf.tlv
vc.param vc.tlv.db.1 vclocal.conf.tlv.0
vc.tlv.db vclocal.conf vclocal.conf.tlv.1
% rm -rf *.*
request virtual-chassis reactivate OR
Reboot with above procedure and check.
Thanks
Partha