if i fully block an attack with spinning blockfist and the attack would deal 0 damage, can i still discard it "before it deals damage" to gain vitality?

Rephrase the question: If I fully block an attack with Spinning Back Fist, can I discard it to gain life?

Yes.

302.8.3.4 If an attack is discarded from the card pool during this step it will still deal its damage during the Damage Step as normal.
302.8.4 The Damage Step: Check the quality of block established in the Block Step. If the attack was completely blocked then it deals no damage. If the attack was partially blocked then it deals half damage rounding up. If the defender decided not to block, or failed to play their block they are dealt damage equal to the damage of the attack. Subtract that amount from the defender’s vitality total.
302.8.4.1 If a block is discarded from the card pool before the Damage Step. Then the attack will still be considered blocked since the block quality was established and the attack took on the completely or partially blocked status during the Block Step.

but the card states that if the attack deals damage, then you can discard it to gain vitality. does that mean that if the fully blocked attack would deal 0 damage, it would still "deal damage" thus allowing me to discard spinning back fist?

No, Aslum is wrong. What he have quoted is right, but irrevelant to the question.

If the attack is fully blocked, it will not deal damage, thus the " before your opponent attack deal damage " doesn't exist, so you cannot use the ability. To be able to use it, you need to take at least one damage.

Exception made if the blocked attack is one that deal half damage when fully blocked ( habitually a throw ).

Same things if you partialy block it with Spinning back fist, then reduce the damage to 0 with another effect.

Before your opponent's attack deals damage, you may discard this card from your card pool to gain X vitality. X equals your opponent's character's hand size. If your character is Cammy, X equals your opponent's character's hand size +2.

The card is discarded BEFORE it deals damage, therefore it's before 302.8.4. The block quality and throwishness of the attack don't matter. It's about to deal damage, this card takes effect before it deals damage. The potential is there. Since it could deal damage that's good enough.

basically, what it comes down to it seems is this,

If an atatck is going to deal zero damage, is it considered "going to deal damage"?

As i understand it an attack dealing zero damage is the same as an attack not dealing damage. Not sure how this works....

It's been stated many times that in order to trigger an effect based on " this attack deals damage", that the attack need to deals at least one damage.

If the attack is going to deal 0 damage, the trigger condition just doesn't exist.

You just cant say " the attack has the potential to do damage" as in comparison with say red lotus trigger "the ability COULD commit my foundation ". Any attack has the potential to deal damage, those effect would simply be triggered anytimes a player play an attack, whatever the condition.

Remember that you can discard this card on any attack, not just the one you blocked it with, but any others attack in the rest of the turn, anytime you are about to take damage.

However, you need to take at least 1 damage.

Vitality loss doesn't count!

This is the same situation as with Holding Ground. because the trigger is BEFORE the damage is dealt, every attack will get to that point and have a time frame where it could deal damage, whether or not the attack's damage is a positive value, so yes, you can use the ability on Spinning Back Fist if the block would cause the attack to deal no damage.

Remember, damage isn't actually calculated until you get to that part of the damage step, which is after the window to respond with Holding Ground or SBF's ability.

Tag, you explained it a lot better then I did.