Dota 2 FAQ - Why is Vanguard core for Spectre?

Why is Vanguard core for Spectre?

bdang94 - You go vanguard if you need the RoH in lane to stay in lane

A naked Radiance is not when Spectre comes on line. Spectre truly comes on line when she has Radiance + 1 tankiness item. You have three options that I think are good.
Drum of Endurance, Vitality Booster, or Vanguard.
You go Vanguard if you need the RoH in lane to stay in lane. This is good against offlaners who can do some amount of damage to you. The sustain and block will also let you farm jungle.
You go Drum of Endurance if you have freefarm and want the stats. It helps you in fights early if you choose to participate.
You go naked Vitality Booster when you have freefarm and want to build it into a Heart of Tarrasque.

 j0a3k - if you're fighting illusion carries vanguard is much stronger

I don't think there's many situations where Vanguard is better than casual  Vitality Booster into later Heart of Tarrasque or casual Ring of Health.
Radiance is where haunt starts killing/zoning supports out of fights, but more importantly it's where your farm accelerates to the point that you can become relevant before all of your barracks are dead. Putting it back several minutes in order to secure an item that drops off in the part of the game where the hero most shines is a bit of a trap.
Of course, if you're fighting illusion carries Vanguard is much stronger...but in most cases I'd rather have the farming item early vs the tank item on spectre. Ideally you won't even fight for the first 25 minutes, and if you do it's with haunt.
That being said, every game is different, and if that Vanguard secures your ability to safely get Radiance instead of dying 10 times to ganks, then by all means go for it.

Soudny - if you have a vanguard, you can always go in woods and farm

I love playing Spectre and I have tried almost all of the builds anyone has suggested only to come to the same conclusion each and every game - get fast Vanguard and you're well set for farming Radiance . A lot of people here basically said the same thing, you don't need babysitter and you can easily farm solo. I have tried purchasing Drum of Endurance, going for solo point booster (which you later transform into Heart of Tarrasque) and, while it was not THAT bad, results have always been better with a Vanguard . Let me just briefly explain why - with Drum of Endurance you will have greater MS and AS, a bit more HP and mana at your disposal. What you will not have is regen which means not being able to stay on the lane after enemy snowballer ganks you and you somehow survive with spectral dagger. It might not sound like a big thing, but trust me, if you hide in the woods long enough while still leeching exp from creeps killing themselves quite close to you - you're being their worst nightmare. Which brings me to my second point - if you have a Vanguard and enemies have shut your farming down by constantly breaking you on lane, you can always go in woods and farm. I know, with solo Vanguard and preferably boots on you, saving 3800 gold to get that relic is gonna take some time, but hey, even if it's slow - still better than moping around in base and whining that you can't farm shit. Your job as a carry isn't simply to farm when it's available because anyone can do that. Your job is to find farm when there is no farm to be found, and that is what differentiates 2k from 4+k.
Even if it might seem a little far-fetched, getting a ring + stout shield into naked boots into Vanguard actually makes you farm Radiance sooner than you would with no items on (since you are then basically a sitting duck)
Hope i helped :)

 Hartwall - Vanguard helps spectre survive from 2-3 man ganks, giving her team more free roaming time

When you pick a  Spectre, you're expecting the game to go really late, which is why you need to make sure that spectre gets all those items. Vanguard helps Spectre survive from 2-3 man ganks, giving her team more free roaming time. A naked Radiance spectre is so easy to gank compared to a VG Radiance Spectre. Basically you make it so that your supports can leave the lane from babysitting you.

 teerre - Spectre has no "core" items. It depends on the game. Vanguard is an option.

What's "core"?

Very few heroes hve really "core" items, Tinker bot is one, for example.
Spectre has no "core" items. It depends on the game. Vanguard is an option.
If you're asking about the default build, no real motive. It coud be anything, they just chose one decent item

 swaglordobama - Vanguard will delay your radiance by about 8 minutes

 It's not core; it's a fallback item if you get shit on. Often times you can settle for a Vitality Booster instead of making a Vanguard , then turn the vit booster into a Heart of Tarrasque later.

Vanguard will delay your Radiance by about 8 minutes. Do not get a Vanguard before Radiance unless you are really getting shut down. If that's the case, skip radiance; go Vanguard , Blade Mail, Diffusal Blade, Manta Style.
The latest you want a Radiance is by the 22 minute mark. It is easily possible to farm Radiance in ~18 minutes if you have safelane freefarm.
If you pick/draft spectre and assume you're going to be up against an aggressive trilane, you should take a support mid with you instead or dodge the trilane and farm in the offlane.

Basically you make it so that your supports can leave the lane from babysitting you.
Not dying is key to farming a relic and Vanguard is a solid not-dying item. Not core but often nice.

Sources: Reddit