Show me where it is defined. You cannot. By contrast, I can show you exactly where "move action," "standard action," "immediate action" and so on are defined.
"Attack action" is not defined.
I just quoted you where it's defined. It's under Standard Actions. It's called Attack. The actions table has Attack (melee) and Attack (ranged) listed under Standard Actions on the table. Look it up and tell me it's not there.
Vital Strike is not triggered by anything other than making an attack (and player choice). Vital Strike is limited by forcing the attacker to make only a single attack.
Uh, no, because Vital Strike forces the user to give up all but a single attack. Did you read the feat?
Vital Strike is limited because it requires the
attack action. The attack action only allows one attack. Nowhere in the feat does it say you are limited in any way. The feat states what Vital Strike does, not what you cannot.
Let's assume for a second that an "attack action" means "an attack," as you claim.
* It's my turn.
* I make an attack as a standard action.
* I glance at Vital Strike. I read the following:
When you use the attack action, you can make one attack at your highest base attack bonus that deals additional damage.
I notice text above that says Vital Strike is a single attack, but that's okay, because I already know that.
* I add my Vital Strike damage to my attack and roll.
* Now, since I believe an attack is an "attack action," I notice that when I use the attak action, I can make one attack at my highest base attack bonus that deals additional damage. I look carefully to see if there is some text that specifies how often I may Vital Strike in a round. I discover there is none. Yay!
* So I perform another Vital Strike action. Now, since I believe an attack is an "attack action," I notice that when I use the attak action, I can make one attack at my highest base attack bonus that deals additional damage....
And so on. Infinite attacks.