Before you slay me like a rabid Gungan, I do realize that this has been explored time and time again... I feel like I just need it explained one more time ?

As far as I understand...

Defense stacks when it originates from different sources. Like armor, weapons, talents... so a character with a shield (deflective 2), an armor (defense 1), and Sixth Sense talent (+1 ranged defense) would have a total ranged defense of 4, right?

Defense doesn’t stack when it is single origin, like two weapons with defensive 1, or an armor, and a personal shield. So a character with a shield (defensive 1), a vibrosword (defensive 1), an armor (+1 defense), and in cover would have a ranged and melee defense of 1, right?

So a player of mine has a Sith Shield (defensive 1, deflective 2) in his primary hand, and a vibrosword (defensive 1) in his off-hand, and in addition he’s wearing and armor (+1 defense). He argues that he would have a ranged/melee defense of 3/3. But I think he would either have a ranged/melee defense of 3/2 or 2/1.

Could someone help me out here? Thanks guys :)

Almost. Items which grant Defense (Defense with just a number after it) don't stack, so Armor (Defense 1) and Cover (Defense 2) would not give you a Defense 3. However, Items or talents which increase Defense (Defense with a + after it) do stack. So, in your example, Armor (Defense 1), Shield (Defense + 2), Sixth Sense (Defense + 1) would indeed grant a Defense 4.

First things first: DEFENSE RULES PER RAW ARE BAD. I would personally just let all defense stack, because that’s what makes sense.

Going by RAW, you have:

Sith Shield grants +1 Melee Defense and +2 Ranged Defense.

Vibrosword grants +1 Melee Defense.

Armor only grants base values of Defense, not amounts that can be added. In this case, I think that value is 1 Defense.

If he’s in cover, that can grant 1 or 2 Defense.

As I understand it: You can only take one source of Defense, but you can take as many additions to Defense as you have available. What important is whether or not there is a plus next to the Defense.

With 0 cover, he has 3/3. (1 base Defense for each, +2 Melee Defense, +2 Ranged Defense)

With 1 cover, he has to choose between gaining Defense from cover and gaining Defense from the armor. It doesn’t actually matter; it’s 3/3 either way.

With 2 cover, he should choose cover as the source of Defense since that is more than the armor. Then, he gets 4/4.

(Defense for cover is a factor that’s decided on by the GM. Most cover is 1 Defense, really good cover is 2 Defense)

Def check the errata for defense so you can show it to your player.

"As many additions to Defense as you have available"... until you hit a cap of 4.

10 hours ago, Yaccarus said:

With 0 cover, he has 3/3. (1 base Defense for each, +2 Melee Defense, +2 Ranged Defense)

With 1 cover, he has to choose between gaining Defense from cover and gaining Defense from the armor. It doesn’t actually matter; it’s 3/3 either way.

With 2 cover, he should choose cover as the source of Defense since that is more than the armor. Then, he gets 4/4.

Cover does not provide melee defense, however. So, in the last case, you end up with 3/4.

16 hours ago, Yaccarus said:

First things first: DEFENSE RULES PER RAW ARE BAD. I would personally just let all defense stack, because that’s what makes sense.

Going by RAW, you have:

Sith Shield grants +1 Melee Defense and +2 Ranged Defense.

Vibrosword grants +1 Melee Defense.

Armor only grants base values of Defense, not amounts that can be added. In this case, I think that value is 1 Defense.

If he’s in cover, that can grant 1 or 2 Defense.

As I understand it: You can only take one source of Defense, but you can take as many additions to Defense as you have available. What important is whether or not there is a plus next to the Defense.

With 0 cover, he has 3/3. (1 base Defense for each, +2 Melee Defense, +2 Ranged Defense)

With 1 cover, he has to choose between gaining Defense from cover and gaining Defense from the armor. It doesn’t actually matter; it’s 3/3 either way.

With 2 cover, he should choose cover as the source of Defense since that is more than the armor. Then, he gets 4/4.

(Defense for cover is a factor that’s decided on by the GM. Most cover is 1 Defense, really good cover is 2 Defense)

I don't tend to play in the upper ranges, but does the game devolve into zero-sum with super high defense and offense? So in other words the pathfinder effect 51/51 = 1/1 so it's the same thing but with bigger numbers so why bother? Honest question and I figure you could answer. Also I think the cover rules aren't great either