you can use the same sticky group in multiple vserver.
Just be aware that if client X is stuck to server Y for vserver Z, the same client X will get stuck to the same server Y for vserver Z' that would be using the same group.
By using different group a client could be stuck to different server depending on the vserver they hit.
Also, if you use the same group in multiple vserver, you need to make sure the same reals exist for the vserver.
ie: if client X gets stuck to server Y on vserver Z, if client X open a connection with vserver Z' then the server Y must be part of the serverfarm under vserver Z'.
Regards,
Gilles.