best way add an irule to every virtual server

First iRules have a priority, default is 500, so edit your iRule event and add a priority lower value than 500 to the events e.g.

when HTTP_REQUEST priority 10 {    

This means the order of the assigned iRules is not important. I highly recommend you assign priorities to all your iRules if ordering is important, which it normally is when you have multiple iRule with the same event in them.

If you have Enterprise Mnager or BIG-IQ I would look to utilise one of these to assign the iRule to all your VSs.

If not then you could script something to loop over each partition on an F5, excluding Common, and add to all VSs. Should be a single tmsh command per partition.

Or you could use Ansible or simply iControl REST and Postman to do the same.