The rules of engagement act

For all VnV Nobles:
All Non Nap worlds are valid targets.

For All VnV Members, not noble:
All worlds that are member of an alliance smaller then 50 (non NAP) are Valid targets, UNLESS a Noble invites you to a group attack.

In general there will be alliance support when an attack backfires, support may be either military or diplomatic. So make sure you can handle the flak militarily yourself.

Probing VnV Members is prohibited, unless you are a member of High Command

You will not probe VnV Alliance members without their explicit permission.