I know he's really supposed to be a counterpick based on enemy team, but I've been picking him for fun in a lot of my recent matches and having surprisingly good results.
I think of my past 10-12 games I've had only 2 disastrous games, the rest have been wins, and often with very solid k/d/a scores.
I played around with a few different skill/item builds and here's what I would consider the "correct" build for almost every situation:
1. Sage's lore (rune fight, u may land this in the fog on the guy running away at 10% hp)
Max glyph, max incantation, ult at 11/16, stats.
Item build almost always:
Starting: Courier, +3str, +3int, runes of the blight
Lane: Blood Chalice, upg courier, ward of sight, ghost marchers
Next: 1x fortified bracer, work on stormspirit, tonnes of wards
I would commonly not complete my stormspirit due to pumping so much $ into wards.
Now some reasoning here...
1. Lore dominance in lane
Nothing annoys/dominates a lane as well as sage's lore. Yes it will kind of suck later on, but for level 1-7 having complete domination in lane and being virtually gank-proof (2 dudes with no mana aren't much of a threat) is huge for you and your partner if you have one.
You never know who you'll wind up against, so they may have an easy way out of sage's lore (hag, panda, armadon, nymph) but the vast majority of the time one or both of your enemies will suffer hugely from being forced to cast their spells.
It will often at a minimum keep the lane pushed in your favour, as pyro uses his wave on the creeps.
2. Team fight dominance through Glyph
A lot of builds suggest 1 point in this early for the int steal, then leaving it til later. I am more with the people who say that at level 9 a 2-3 second silence is devestating in team fights. This can vitally screw a lot of combo chars (pyro, pebs, panda) and make your lane that much safer from ganks - as well as neutering such enemy chars in team fights.
3. Late game DPS through Incantation
A lot of supports will wither by the time level 17 or so rolls around. Vindi is the opposite, turning into almost a semi-carry with master's incantation. The reason I don't get this earlier is that it rapes your mana pool (its 15 mana per use at all levels) for pathetic damage increase (its like +8 at lvl 1).
Now you CAN use it to harass enemies in lane without drawing creep aggro, but I've never found this necessary. I've tried several times just to see if it was worth it and it made no difference. Drained my mana reducing what I had for curses, did pathetic dmg increase. You can harass without getting owned by creeps if you play properly - don't waste a skill point to be lazy.
Usually by the time I've got this to level 4 I'm hitting +40/50 true damage. If the game goes long and my farm is on it might go to +80 or even +100 (kuldra/frostfield).
While you still wont go toe-to-toe with any decent carry, in team fights after you've silenced and cursed you don't have to sit there with your thumb up your ass. You can contribute some pretty decent damage. I often find I will FC/Curse, run into fog so enemies don't focus me, then right back into range and start wailing on them with MI.
4. You are the Support
I really think a lot of the success I've had recently with vindicator is from spending ridiculous amounts of gold on wards. People generally don't want to do this because it doesn't directly increase their stats and seems like a "waste".
I find vindi is fun enough to play that its a small sacrifice, and being one of the squishiest of squishies I am really helping my own cause here by seeing the ganks come in.
5. Sell blood chalice by mid-game
This is only for curse spam in lane. Once lane is done you will find you don't need this anymore. Buy a manatube (towards kuldra/stormspirit) and chuck the chalice.
6. Stormspirit is awesome
The disable is neat of course, but the movespeed with the disable will allow you to get away from any 1v1 situation you don't like. It will also allow you to catch someone who is running and toss them in the air until your team catches up.
Vindi is slow as hell, which is why I advocate ghost marcher over steamboots. Stormspirit goes even farther to solving his lethargic pace.
7. You can mid
I've only done it a few times and mostly vs bad players, but you could be the right choice for mid vs heroes like deadwood or pebbles. It's possible to gank with vindi but you need good coordination... your lane has to jump in first, absorb a few spells, then you come in with curse and autoattacks. If you open with curse you'll accomplish nothing so you can't actually initiate the gank.