can an irule call another irule

If I may add, procs would definitely be the way to go here. I might have also said that a sideband call might work, but that too requires v11. There still may be a way though, depending on how the main rule interacts with the VIPs, and at what (if any) event level it needs to fire. So lets say for example you have a main rule that gets triggered at HTTP_REQUEST, and your two almost identical iRules, which probably also trigger code at HTTP_REQUEST. You could add the main rule to each VIP and use a few conditions inside that iRule to determine when and what to run. For example:

VIP iRule:

when HTTP_REQUEST {
    ...doing something important here
}  

Main iRule:

when HTTP_REQUEST priority 501 {
    switch [virtual] {
        "app1-vs" {
             do something for app1
        }
        "app2-vs" {
             do something for app2
        }
    }
}

In this way, you have a single main iRule that 1) gets triggered after your VIP-specific iRule, and 2) evaluates code based on the caller VIP.