Hi! I have some questions
When attacking and adding the vengeful blade, If I have Weapon Mastery (let's say shortsword), will it activate also the vex?

And also, if I have "Dual Wielder" feat, and connect an attack with the light property which I'm also using the vengeful blade on the attack, will I be able to make the attack as a bonus action? Or no? 
Thanks for the help

I don't have that book, but I think, after reading this post, it's similar to Booming Blade or Green-Flame Blade. If I'm right, then you can use the Vex property with Vengeful Blade.

Regarding your second question, Dual Wielder feat requires to take the Attack action (see below) and you usually need to use the Magic action for casting the cantrip. However, you could do what you need thanks to, for example, Eldritch Knight's War Magic, College of Valor Bard's Extra Attack, or Bladesinging's Extra Attack, to replace one of the attacks with a casting of Vengeful Blade.

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Enhanced Dual Wielding. When you take the Attack action on your turn and attack with a weapon that has the Light property, you can make one extra attack as a Bonus Action later on the same turn with a different weapon, which must be a Melee weapon that lacks the Two-Handed property. You don’t add your ability modifier to the extra attack’s damage unless that modifier is negative.

We were talking about Vex, but in reality, any other Weapon Mastery could be used with Weapon Cantrips. The only questionable property is Nick, as debated here: Questions regarding Shillelagh, War Magic, True Strike and Extra attacks 

Also, let me add here some related threads in case they're useful for you: 

• True Strike and Dual Wielding 
• True Strike and Weapon Masteries 
• Shadow Blade 
• 2024 Weapon Mastery and Two-weapon Fighting with Weapon-based Cantrips 
• Does weapon mastery work with a spell attack action