I'll basically repeat what G___R___E___E___D said, except with more details.Stacking damage block works the same as stacking crits or bashes - you can proc multiple damage blocks (chance stacks multiplicatively), but last acquired overrides others. So with multiple vangurds full calculation for proc chance would go like this: (since we ony care IF it proc or not)Chance you proc vanguard : 0.7Therefore chance you won't proc it: (1-0.7)Chance you won't proc any of your vanguards: (1-0.7)*(1-0.7)*... etc. depending how many you got.Now opposite to above - chance you proc at least one (or more):1 - (1-0.7) = 0.7 //if 1 vanguard carried1 - (1-0.7)*(1-0.7) = 0.91 //if 21 - (1-0.7)*(1-0.7)*(1-0.7)= 0.977 //if 3 etc.So you can see chance barely changes with more than one.Now last acquired overrides others which means amount of damage blocked does not increase.However multiplying chance by damage blocked you can calculate how mych would you bock on average, and get (for melee):28 for one36.4 for two39.08 for threeParticularly revelant, Tidehunter's Kraken shell also uses damage block (100% chance, 28 dmg), so if you skill his passive then get vanguard:Chance any of then will proc is 1 (because kraken would always work, duh)Vanguard is last acquired, thus overrides kraken on proc.If vanguard proc, you get 40 damage blocked with 0.7 chanceOtherwise it's 28 blocked via Kraken with 1-0.7= 0.3 chanceEffectively blocking 40*0.7 + 28*0.3 = 36.4 blocked on average (30% increase compared to 28 from kraken)You can do similar calculations for PMS VG on your own, just remember about reverse order of acquisition.Regarding magic resistance. Magic resistantance stacks multiplicatively, and calculation would be similar to 1st equation with vanguards:Let's assume you have similar sources of magic resistance (no immunity, no barrier) with values of A, B, C, ... respectively.After applying magic resiatance A, damage will be reduced from 1 to (1-A), etc for othersAfter applying all of them, damage will be reduced from 1 to (1-A)*(1-B)*(1-C) ... etc. (multiplicative stacking)Now, magic resistance would be difference between damage before and after reduction:1 - (1-A)*(1-B)*(1-C) ... etc. (see the analogy with damage block chances?)In place of A, B, C... you can put:- innate hero magic resistance (0.25), - Magina Spell shield (0.26/0.34/0.42/0.5), - Planeswalker cloak, Hood or Pipe (0.15 or 0.30) warning, only last acquired will work if you get mutiple- Null field (0.05/0.1/0.15/0.2) - Flesh Heap (0.06/0.12/0.18/0.24)- Ice Vortex (-0.1/-0.15/-0.2/-0.25)- Veil of Discord (-0.25)- Natural Order (-0.06/-0.14/-0.2/-0.26)- Ethereal state (-0.4), though it technically does not affect magic resistance, but all in all calculation is the same.Negative values for amplifiers.Play around with numbers to see how ineffective it gets if you start stacking too much.
Magina's resistance is COMPLETELY stackable.Thus making your magic resistance 80% (if hood acquired)