Vengeful Spirit is a good Disabler and Support though he can be a Hitter/Carry because of his attack damage. When playing as a support, better go around the map and gank enemies together with the other support. When chose as a support, his item build should be supporting items but when chose as a carry, his item build should deal more on attack speed. Though I prefer Vengeful to be a support, as this item build.
The early game item build for Vengeful Spirit can be the following:
Ironwood Branch – This item grants each attributes for Vengeful Spirit giving him more strength and toughness to survive the early game plus the intelligence for mana support.
Ancient Tango of Essifation – This item helps Vengeful Spirit to regenerate when in mid health condition.
Healing Salve – This item helps Vengeful Spirit to regenerate more fast whenever he is in a low health condition.
Magic Stick – This helps you to counter spell caster enemy heroes.
Observer Wards – This is a strategic item to spy on your enemies. Supporting heroes just like Vengeful Spirit should do the warding strategy. This should maintain through the whole game.
Clarity Potion – This item helps Vengeful Spirit to regenerate his mana whenever it is in low condition.
Smoke of Deceit – This supportive item helps Vengeful Spirit and his allies to set a clash or gank against his opponents during the early game. This should be maintain through the whole game.
Sentry Wards – This strategic item helps Vengeful Spirit to deward a spying observer ward nearby.
Boots of Speed – This grants Vengeful Spirit movespeed for chasing and escaping.
The mid game item build for Vengeful Spirit can be the following:
Headdress of Rejuvenation – This may help Vengeful Spirit for better health regeneration.
Medallion of Courage – This can do the killing faster because of armor degrading.
Urn of Shadows – This can help Vengeful Spirit to serve himself and his allies in health regeneration. And also can do damage to enemies.
Magic Wand – This helps you to regenerate your mana and health in an instant depending on its charge.
Arcane Boots – This grants Vengeful Spirit and his nearby allies mana whenever its cooldown.
Ancient Janggo of Endurance – This grants Vengeful Spirit and his allies attack speed and movespeed.
Necromancer – Its ability to summon two useful creature can plus his skills can be a good partner in killing enemy heroes.
The late game item build for Vengeful Spirit can be the following:
Mekansm – This provides Vengeful Spirit and his nearby allies armor and hitpoints.
Khadgar’s Pipe of Insight – This helps allies of Vengeful Spirit and himself to prevent magic for damaging more.
Force Staff – This helps Vengeful Spirit in chasing and escaping.
Eul’s Scepter of Divinity – This grants Vengeful Spirit the ability to cyclone enemies and himself for chasing, setting and escaping.
Dagon – This grants Vengeful Spirit an ability to instant kill an enemy.
Aghanim’s Scepter – This grants him a new stunning ability.