Can you combine Vital Strike and Felling Smash?

Vital Strike (Combat)

Benefit: When you use the attack action, you can make one attack at your highest base attack bonus that deals additional damage.

Felling Smash (Combat)

Benefit: If you use the attack action to make a single melee attack at your highest base attack bonus while using Power Attack and you hit an opponent, you can spend a swift action to attempt a trip combat maneuver against that opponent.

Can these 2 feat be combined?

I don't see why not. Felling smash looks like it has crazy prereqs though. What does Combat Expertise have to do with tripping?

Yes, they can be combined. Both are simply add-ons to the standard Attack action. Combat Expertise isn't required to Trip someone; it's required to be able to Trip someone while keeping yourself out of harm's way.

Is it? Its a feat tax as far as I can tell. Yeah, vital strike is a standard for one swing and felling smash is a swift while making one swign and using power attack. So I don't see why they shouldn't be used together.(beyond vital strike being meh if you can full attack, but thats another conversation altogether)