I try to create a homebrew bard subclass feature, I already got idea for 3rd level and I need your advice on this ability (is it broken or to weak?).

So here is the feature.

Extra Cantrip

You learn Vicious Mockery as your cantrip, if you already have this cantrip you can choose another cantrip in Bard spell list.

Flyting

You present your poem out loud in the public to insults a creature. For 1 minutes you spend to read your poem loud and clear, you can choose one creature that you can see and can understand you. You make a Charisma check against a creature you chose, contested by the creature Charisma check. If you succeed, the creature will have disadvantage on all ability check, attack roll and saving throw as long as the creature can see you.

Alternatively, you can use this feature to motivate your friends and allies. You can choose a friendly creature and they have advantage for all ability check for an hour.

Once you use this feature, you can't use it again until you finish a short or long rest.

*Not Naming This Feat Yet*

You are the master of the word play, you know how to shorten a sentence. As a bonus action, you can expend one use of your Bardic Inspiration to cast Vicious Mockery, it will deal extra damage equal to bardic inspiration die + Charisma modifier. When the targeted creature succeed the Wisdom Saving Throw it will receive half of the damage. 

That is, sorry for my broken English.

Update note:
20th Oct: Change from "As a bonus action, you can expend one use of your Bardic Inspiration to cast Vicious Mockery. When the targeted creature roll for Wisdom Saving throw for the Vicious Mockery, roll bardic inspiration die and reduce the number of the saving throw." to current version.
22nd Oct: Add Flyting ability 

I think it's a little weak. Vicious Mockery is a neat and thematic spell, but it's not really that strong. Maybe instead when you cast Vicious Mockery you can expend one of your Bardic Inspiration dice (no action necessary) and your Vicious Mockery will affect up to a number of additional targets of your choice in range, besides your original target, equal to the result you get on your Bardic Inspiration die.

I'm actually working on a subclass that involves something like this too. I went a different route by having the main feature focus on replacing the effect that VM does then having the inspiration feature focus on doing something else and not tying it to VM. Now what you're doing can work, but I do agree with what Ophidi said because personally I think that VM is actually an underpower spell, even if it does psychic damage. It needs something else to help boost it.

Also change it to "you learn the vicious mockery cantrip" wording it like this makes it more aligned with how WotC has similar features. You don't need the "if you already have this cantrip, you can choose another cantrip in Bard spell list," because when you get to level three, you can also change vicious mockery for another bard cantrip if you know it already already, but nothing wrong in including it if you want.

Owh I see, so casting a cantrip that cost action using bonus action is not that powerful. Thanks for the insight, I will use alternate version instead.

Quote from ShadowJoker767 >>

Owh I see, so casting a cantrip that cost action using bonus action is not that powerful. Thanks for the insight, I will use alternate version instead.

I mean, it make it more powerful, but it's still kind of weak to build a class feature around. Burning a Bardic Inspiration die to cause a little bit of damage is usually not the best way to use it.

Quote from Ophidimancer >>

I mean, it make it more powerful, but it's still kind of weak to build a class feature around. Burning a Bardic Inspiration die to cause a little bit of damage is usually not the best way to use it.

After you highlighted that in first comment, I change it a little bit to increase the damage rather than reduce the saving throw.
1d4+bardic inspiration die + Charisma modifier, succeed saving throw will deal half the damage.

The idea for this bard is to enable it to cast spell quickly. work is in progress, as you see the feature still didn't have it name yet.

Honestly, I would take a look at what other creators have done with similar homebrews that focus on VM... We need to know a bit more about what you're intending for the subclass. Check out the College of Quips homebrew, I think its a fairly good example of a bard subclass that focuses on VM, though I do have my own grips about it.

Personally, I try to avoid making bardic inspiration features main feature of the subclass because then it kind of forces you to either balance the subclass based on a short rest feature or requires you to add an additional feature, like with the college of swords, where you can use a baseline die instead of needing to use the bardic inspiration. And as I said, I'm also making a subclass that focuses on vicious mockery as well, and found other ways to buff the spell without having to rely on spending the bardic inspiration for the occasional damage buff.

The theme suppose to be poetry and wordsmith, since you are expert with wording you know how to create meaningful sentence (example idiom) with less word. Thus you able to shorten the casting time spell that have verbal component, such as:

• action using bonus action
• ritual spell doesn't need extra 10 minutes
• etc

Currently in my head:
3rd lvl only Vicious Mockery can be cast using bonus action and get extra damage by spending 1 bardic inspiration
6th lvl ritual spell that have verbal component doesn't need extra 10 minutes | can cast vicious mockery using bonus action + extra d6 damage twice per long rest without spending bardic inspiration
14th lvl can cast any attack and buff spell that have verbal component and target single creature with bonus action, can use this feature a number of once per long rest, you can use again using bardic inspiration.

this is work in progress so there will be a lot of updates later on. thanks for the highlight, I might add additional feature for 3rd level later on.