Can you use Vital Strike with Focus Attack? Using a Rapier and having either Keen Enhancement or Imp Crit
Vital Strike make 1 attack at highest BAB deal double weapon dice in damage
Focused Attack (Combat)
Prerequisite: Base attack bonus +6.
Benefit: When you take a full attack action, you can choose to make only one attack. This decision must be made before making your first attack. For every attack you could make but don’t, the critical threat range for the one attack you make increases by 1. This increase happens after all other adjustments to your critical threat range. Thus if you could make three melee attacks with a keen scimitar as a full attack, when you use Focused Attack you take a full attack action to make a single attack with a threat range of 13–20 (18–20 for a scimitar, 15–20 for a keen scimitar, 13–20 for giving up two attacks with Focused Attack and a keen scimitar).