Vendetta is a generic Shadow Walk ability (works identically to the one from the item SB or the one BH has) – it turns Nyx invisible for a period of time and gives him bonus damage the first time he attacks while Vendetta is active.
Offensive Use:
The spell is your main initiation tool and the high physical nuke damage is essential for getting solo kills, especially early on. Your kill combo usually consists of Vendetta hit into Impale into Mana Burn and attacks to finish the kill. This will give you the maximum possible nuke damage, however, using the Vendetta hit first will make landing your Impale harder (it will give your opponent time to react, change direction to dodge or simply use an escape mechanism).
In the ideal scenario, an ally will be able to use a disable to set up for your stun. If you are the initiator, however, you will have to make a decision whether to hit out of Vendetta or to use the stun right away (which is the more reliable initiation but decreases your nuke potential a great deal).
The rule of thumb is that if you have allies backing you up that have enough damage to secure the kill, it is better to use Impale directly out of invisibility. If this is not the case and you need your nuke damage to secure the kill you wouldn’t have the luxury to use directly the stun and you will have to try to execute the full combo.
The target you are focusing could also help you decide whether to go for the full combo or for the safer option. Generally speaking, it is better to directly use stun on targets with a BKB, Blink or other fast escape mechanisms. Also, it is better to initiate on targets with instant disables from behind (this way their turn rate will give you the needed time to cast Impale).
If you are initiating into multiple opponents activating Spiked Carapace could keep you safe for a while and disallow them to effectively counter-initiate.
You need to keep true sight in mind and always check the inventories of opponents before you go in – you can easily get yourself killed if you walk into enemies that actually see you.
Defensive Use:
Vendetta amplifies the MS of Nyx which is useful for chasing down targets or running away. The spell also has an instant cast animation and a fade time of 0 seconds, which makes it a very powerful survivability tool, provided your opponents have no true sight.
Duration and CD:
Initially, the spell has a relatively long CD and usually, you would want to use it only for kill attempts – if you are forced to use it as an escape mechanism you will have to wait for the CD to go on the aggressive. As you level it up, however, the duration of the spell increases and the cooldown decreases. At lvl 18 the duration will be greater than the CD – this theoretically allows you to stay invisible all the time.
If you initiate with Vendetta in the final few seconds of the duration you will be able to use it again almost immediately. If you activate it and use it right away, however, you will have to wait the whole 50 seconds CD. Because of this, it is a good idea to move around the map inside Vendetta so that when you find a target and initiate there wouldn’t be a lot of time left on the CD.