Irule persist lookup source_addr for specific virtual server

Unless 'Persist Across Virtuals' is set then the persistence is per virtual server anyway. If it is set then yes, you can control this with the command as per https://devcentral.f5.com/wiki/iRules.persist.ashx

persist lookup   [all|node|port|pool]  
"all" or no specification returns a list containing the node, port and  pool name.  
Specifying any of the other return types will return the specified item only.       
 =  | {  [any virtual|service|pool] [pool ] }        
the latter key specification is used to access persistence entries across virtuals, services, or pools.

Marvin,

For the most part, getting an added command to look across other virtuals persist records without enabling persist across * is slim. There are probably too many dark corners around which that could / would break. The preferred mechanism to do this would be with the table commands.

A minimal framework using the table commands could look similar to this:

when RULE_INIT {
     probably makes more sense to use data groups here
     quick and dirty PoC
    set static::web1_peer "/Common/vs_web2"
    set static::web2_peer "/Common/vs_web1"
}

when LB_SELECTED {
  table set [virtual name]:[IP::client_addr] [LB::server]
}

when HTTP_REQUEST {
    if { [virtual] eq "/Common/vs_web1" } {
        set peer ${static::web1_peer}
    }
    elseif { [virtual] eq "/Common/vs_web2" } {
        set peer ${static::web2_peer}
    }
    set peer_persist [table lookup ${peer}:[IP::client_addr]]
    if { ${peer_persist} ne "" } {
        log local0. "${peer} persist-record: ${peer_persist}. Select and persist to partner memeber."
    }
    else {
        log local0. "Peer persist-record not found.  Making selection."
    }

}