This is one possibility out of many: tr -s "[,'" "\n" | grep "^ip=" | cut -d "=" -f2
Example:
echo "vif = [ 'ip=1.2.3.4, mac=00:00:00:00:00:00, bridge=eth1', 'ip=5.6.7.8, mac=11:11:11:11:11:11, bridge=eth1', 'ip=9.1.2.3, mac=22:22:22:22:22:22, bridge=eth1' ]" | tr -s "[,'" "\n" | grep "^ip=" | cut -d "=" -f2
produces
1.2.3.4
5.6.7.8
9.1.2.3