I don't even understand the Vulnerabilities system neither,
Is "minor vulnerability" something i should focus on, or should i focus on core/significiant vulnerabilities?
Minor vulnerability is what the girl care about the least, while core is what she treasures the most. Breaking minor before core on same vulnerability will make both girls in question friendlier with each other, while breaking core before minor is a bit harder, but will tense things up between them. Friends will come to the battle faster, while rivals will take their time. Also if relationship switches from friendly to hateful you will get double evil energy for that break.
Though it doesn't really matter until final vulnerabilities - only they will decide whether girls end up as unbreakable friends or bitter enemies. Thats where you shouldn't fuck up.
Friendliness may have some use if both of them are converted to forsakens, but otherwise you will probably want them to loath each other. It will make final showdown a LOT easier.
Significant vulnerabilities are not a part of this system and it doesn't really matter when they are dealt with. Also if core vulnerabilities broken early, they will give bonus evil energy during scheduled public events on days 15 (tier 1), 30 (tier 2, or, possibly, 3?) and 45 (tier 4).
Even "openings" i hardly understand,
To make an "opening" and be able to surround a character.
Do i need to raise a trauma up to a certain level, or are multiples lower lvls is enough?
To make openings you need trauma. Curcimstances, however high, don't count. Multiples of lower level are enough , but keep in mind conditions, under which trauma becomes the opening:
Fear applies only if some other chosen is surrounded or captured.
Disguist always applies, but only half of its total level (rounded up).
Pain also don't have a condition to apply, but after being used in surrounding it stops working until its level is raised.
Shame is a prolongator - it doesn't make openings on its own, but gets added to total number when other three traumas make enough openings to beat target's defense.
Let's say a girl need a lvl 3 def opening,
Do i need to raise a Pain-lvl 3, or is having a DIS, PAI, SHA all lvl 1 is enough?
Pain 3 will work in that example. Disg/Pain/Sham 1 will not, because total sum without sham is less than 3. You will need either fear/disg/pain 1, or disg 3 (remember, half of number) and pain 1, or disg 1 and pain 2 to generate 3 openings.
i'm not even sure if i'm meant to focus on doing Trauma-dmgs,
Or if i'm meant to focus on making Circumstances-dmgs.
You need both. Trauma is the resourse you after, but to make any meaningful amount you will need curcimstance multipliers.