redirect based on port

You dont even need an iRule for this. A virtual server on the LTM can be an IP/port combination. Simple create 2 or more virtual servers listening on the ports you need, and attach the required pool to each. The destination port with be 'redirected.'

for example : vip --> server a vip:8081 --> server 2 or for example : vip --> server a vip:8082 --> server 3

create an individual VIP per port required. Same IP, different port, and allocate port in pool setup.

thats exactly ehat i did and it does not work when i add the port to vip it is going to nowhere

create an individual VIP per port required. Same IP, different port, and allocate port in pool setup.

Apply the entry port to the VIP (I will assume standard http) , create the pool with the backend server(s), associate the port at the pool level, then it will go from HTTP > Whichever port is associated with pool.