Assume you use the agent user (just the way to use for future purposes).
If ACD user is not logged in, ie 553. You need to have rerouting setup for 5531, if you want this number to go anywhere. If you login to the ACD path with 5531, 553 becomes unavailable unless you set rerouting/divert to a specific destination. So 553 will go to vm final, as the agent id logs into the ACD path (553 will become unavailable, with fixed divert to vm). You might set divert on 5531...but that's another vm user (ie 553 vs 5531).
My (humble) advice. Have the 553 routed to vm, once the agent logs into the ACD path. Than take care of rerouting/divert on the 5531 number. Could be the same as for the 553 number EXCEPT routing to vm will end up in 2 different vm box'd...one for 553 and one for 5531. Depends a little how you want to stick to the old situation or change towards the current capabilities.