How to create iRule to grant additional access to a specific client

How would an iRule help here? Is there currently one in place that restricts access?

If I may add, authentication, or some form of identity validation is required for this to work. If you validate the request as coming from a particular vendor, either through an authenticated session, or perhaps a known IP range, then the iRule would be pretty straight forward. Example:

when HTTP_REQUEST { 
    if { ( [class match [string tolower [HTTP::uri]] starts_with my_uri_dg] ) and ( $vendor equals "my_special_vendor" ) } {
        return
    } else {
        drop
         or
         HTTP::respond 200 content "...no access..."
    }
}

where "my_uri_dg" is a string-based data group with the URIs you want to protect. Example:

/customerinformationservice/v1/customerinformationservice.svc := 1 
/productservice/v1/productservice.svc := 1

You could optionally define which vendors had access to specific URIs. Ultimately though,

and ( $vendor equals "my_special_vendor" )

you have to figure out how to differentiate one HTTP request from another.

Hi...I am new to iRules so my iRule construct is a bit basic, but here is the iRule

when CLIENT_ACCEPTED { if { [[IP::remote_addr] equals 203.45.160.26] || [[IP::remote_addr] equals 203.45.160.28] || [[IP::remote_addr] equals 203.45.160.31] } { pool bat1_test1 } else { pool bat1_test2 } }

Basically the idea is if the source IP address is from the 3 specific addresses it should sent the traffic to bat1_test1. All other traffic should go to bat1_test2. The question is the Virtual Server (VIP) has only one default pool bat1_test1, so I don't know how the irule will divert to bat1_test2 pool. When I applied the iRule, the normal traffic (i.e. access to bat1_test2) also stops working

I'd suggest you change the logic a little and use a data group to keep the code simple. I'd make bat1_test2 the VS default pool and use this iRule after creating a DG call 'allowed_ips' with those three addresses in it;

when CLIENT_ACCEPTED { 
 if { [class match [IP::client_addr] equals allowed_ips] } {
  pool bat1_test1 }
}