F5 BIG-IP answers with a self-ip that is not associated with that VLAN

YOur problem description is a bit unclear, but I'd like to point out a few things.

The BIGIP is unlikely to be responding to Requests from your proxies. Your Proxies should be sending Connections to a specific IP address on the BigIP (a Virtual Address most likely), and so the response to the proxy will be the IP of the virtual. If you see some traffic from the Bigip to the Proxy and it is using a floating IP address, then it is likely to be traffic originating from a client, and also handled by a Virtual server. If you see traffic with a self-IP, then it is most likely to be monitor traffic (and you should probably ignore it).

You will need to describe better what objects are involved, and how the traffic flows.

Example:

Ingres ->vlan internal -> Virtual vsinbound ->Egres: vlan proxy; -> Pool: proxy-pool

Then paste the config of the virtual.

Do this for all the relevant flows, and it should present a better picture what's going on...