My group and I have recently ran into an issue with the spell Enlarge Person and the feat Vital Strike. We have two conflicting viewpoints based on a line from Vital Strike that states the following:
When you use the attack action, you can make one attack at your highest base attack bonus that deals additional damage. Roll the weapon’s damage dice for the attack twice and add the results together before adding bonuses from Strength, weapon abilities (such as flaming), precision-based damage, and other damage bonuses. These extra weapon damage dice are not multiplied on a critical hit, but are added to the total.
And the bigger issue arises with the following line from Enlarge Person:
All equipment worn or carried by a creature is similarly enlarged by the spell. Melee weapons affected by this spell deal more damage (see Table: Medium/Large Weapon Damage). Other magical properties are not affected by this spell. Any enlarged item that leaves an enlarged creature’s possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown and projectile weapons deal their normal damage. Magical properties of enlarged items are not increased by this spell.
The argument goes like this: Enlarge Person is a spell that is providing damage to a weapon based on its increased size. However, it does not say if this is simply a bonus damage effect or an increase to the weapon's base damage. If we are to interpret the term "base damage" as "damage dice that your weapon deals without modifiers or added effects", then Vital Strike should only ever double the damage dice of a weapon provided at its original size category and not the damage dice after the Enlarge Person spell is in effect because it does not fit the definition of "base damage."
We also realized that many effects and feats say to "treat the damage dice as though the weapon were one size larger", and that only complicates matters further as to the proper application of Vital Strike when it comes to weapons modified by effects of this sort.
So, to boil it all down, I ask these questions: What is the proper way to handle Vital Strike's double damage when weapons are increased by a size category or treated as such? Is there a way to explain this using RAW?