[2.6] Theorycraft. Scion Vampire mage

Hello guys. I'm kinda new to poe and I have an idea to build kinda vampire mage with Scion.

So far I'm thinking to base the build on these milestones:
- slayer ascendancy
- occultist ascendancy
- vaal pact keystone
- ghost reaver keystone
- chaos inoculation keystone

I have some questions:
1) is leecher mage a viable build in poe?
2) does slayer's 10% overkill leeched as life transforms via vaal pact and ghost reaveer to shield? (if not, then i'd have to get rid of either reaver or slayer)
3) the primary damage source of this build is planned to be some kinda mix of curses and some heavy magic damage spell. Can you give me a piece of advise on the most efficient spell gems?

If the life leech bonus from reaver is not worth it, I probably think to switch the character class to witch as there's nothing of use but reaver in scions ascendancy classes (berserker's 1% seems miserable imo)

I suggest playing the game with a few builds more experienced players have made before theorycrafting anything. This game is too complex to create a successful build that can survive mid-end game content if you have virtually no experience. Scion is already one of the trickiest classes to create a build from, I wouldn't start there.

1) "Leecher Mage" isn't the way people think about spellcasters that leech in POE. In POE, you MUST have one of the following types of life/ES sustain: Leech (Life/ES), Regeneration (Life/ES), Recharge (ES), Life Flasks (Life). So yes, leech is "viable".

I'm not sure where you're going with the idea of "mage", but dump that "box" and think outside of the box. Most builds aren't necessarily pure "mage", "archer", or "warrior" in this game. You have Marauders that summon totems that attack with your weapon, Occultists that cast melee range spells that persist around you while wielding a bow (dafuq?), and Tricksters (sneaky rogues) that spin while wielding daggers that explode with tons of flashy spell damage when they hit stuff with a dagger (not that sneaky). Skills that are used as a build's primary form of attack are generally classified as an "attack" or a "spell". So yes, "spells" are viable.

2) Yes and yes. When you a kill a target with that ascendancy node, the game sees the extra damage past the point of the target's life pool, takes 10% of that, and gives it to you as life by default. Vaal Pact and Ghost Reaver modify leech (making it instant instead of over time, and giving it to ES instead of life) regardless of source AS LONG as it is keyworded by LEECH. If it does NOT say "leech", then Vaal Pact and Ghost Reaver do nothing (life/mana on hit from certain claws, Life on Hit Support gem, jewels, Poacher's Mark, and life/mana on kill effects are NOT leech effects).

3) Primary Damage Source is generally going to be whatever attack/spell you put into a 6-link chestpiece or 2 handed weapon. Curses generally only enhance that damage and will not be a "primary damage source" for the simple fact that by the time you get to end game content, bosses and map mods will significantly weaken the effects of curses or ignoring them outright. Map bosses have 60% less curse effect, Guardians/Shaper have 80% less curse effect.

I strongly, strongly suggest you look through several build guides to see how people design the offense and defense of a build before attempting one yourself to familiarize yourself with how the game works. It'll save you a huge amount of time and frustration trying to learn the game. My recommendations for new players:

1) Play a LIFE based character to start. These are more straightforward to gear (Chaos Innoculation builds generally require you to start with life nodes/gear, then switch to more expensive ES gear around 60-75, tricky for new players). You can use life flasks to easily sustain your life pool, as opposed to ES, which requires more careful playstyle/build mechanics to sustain. Marauder is one of the easiest to start with given the straightforward build tree and high survivability.

2) If you want to play mainly with spells, recommend Assassin (for physical/poison spells such as Blade Vortex or Ethereal Knives), or Inquisitor (elemental spells). Both classes can build life more easily than Witch and have far more optimized specialized ascendancy nodes for spells than Scion.

Once you understand the basics of what makes a successful build, then you can try theorycrafting. Here's some basic rules:
1) You must have LAYERS of defense. You MUST have a large life/ES pool (5-6k life or 8k+ ES minimum at level 90). You will want to pair that with curses (i.e., enfeeble/temporal chains), armor, evasion, dodge, block, endurance charges, physical damage reduction, fortify, defensive flasks, or some combination of these. Then you need to decide on a sustain mechanic (that I listed above when talking about leech).
2) You will pick ONE primary damage skill, whether it is an attack or spell. Plan out your support gems for that skill in a 6 link setup (i.e., Flameblast - Spell Totem - Increased Critical Strikes - Controlled Destruction - Faster Casting - Increased Area of Effect).
3) Pick out some 3L or 4L support setups and auxiliary gems. Popular inclusions include mobility skills (i.e., Shield Charge - Faster Attacks - Fortify), auras/mana reservation skills (Hatred, Herald of Ash, Arctic Armour, <Curse on Blasphemy>), defensive reactions (Cast When Damage Taken - Immortal Call), offensive supports (Spell Totem - Wither - Faster Casting)

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Lots of information, but POE is just that complex. This is nowhere near all the information you need to know, just the tip of the iceberg. Hopefully it gets you started though. Good luck! Just remember if you feel stuck, go read/watch some guides or watch some streams. The popular ones (ZiggyD, Mathil, LiftingNerdBro, and many others) all have guides and gameplay on youtube/twitch you can experience to become successful in the game.