It looks like that you are using a BCC+ FX for the VHS effect. This VFX should have a Mocha masking option in which you can mask the VHS effect to the screen area of the TV and thus prevent the VHS effect spreading beyond the TV screen area. Given that the TV screen shape is a fixed still image, Mocha tracking shouldn't be necessary other than saving in Mocha before exiting from Mocha back to the BCC+ VFX.

@Dexcon  Hi, BCC+ FX? What is uni.VHS? is it Red Giant?

@Gid

What is uni.VHS?

No idea. I'm currently in Europe with a small laptop and only saw the BCC+ FX. Thanks for posting an enlarged image of the FX screen - my suggestion is clearly not an approach if a BCC VFX was not being used.

It looks like that uni.VHS could be a Maxon Universe FX.

@Dexcon I'm in the UK but I have a a 4k monitor 😉 Either way it looks like the OP needs to mask the fx to limit it to just the area required.

 Either way it looks like the OP needs to mask the fx to limit it to just the area required.

Yes. Agreed.

@Gid

What is uni.VHS?

No idea. I'm currently in Europe with a small laptop and only saw the BCC+ FX. Thanks for posting an enlarged image of the FX screen - my suggestion is clearly not an approach if a BCC VFX was not being used.

It looks like that uni.VHS could be a Maxon Universe FX.

Thank you very much for your answers.

Yes uni.vhs is an effect from Maxon Red Giant "Universe". I'll give you a screen of the plugin settings, I don't see an option to adapt the effect to the reduced size of my footage, it continues to be displayed on the entire screen.

Afterwards I used this plugin but if I used another the problem would be the same for example if I applied a lut the black background around the TV is impacted and is much less black.

For now, the idea of ​​using a mask as recommended for the effect could work. But for my VHS type plugins if I use a mask I risk removing information and reducing the extended range of the effect, what I would like is to apply the effect to the scale of my footage. Otherwise I could prepare it in advance for each shot but I'm afraid of a loss of quality by encoding so much. Someone recommended the technique on Photoshop but afterward I am not only going to use still images in the TV but also videos.

I haven't yet been able to test all your suggestions, I'm going to do it this am and get back to you but I wanted to give you this additional information on the plugin used.

@davewire35 Hi.

For now, the idea of ​​using a mask as recommended for the effect could work.

It will work.

But for my VHS type plugins if I use a mask I risk removing information and reducing the extended range of the effect, what I would like is to apply the effect to the scale of my footage.

I'm not entirely sure what that means but it looks like you've used Pan/Crop to reduce the size of the image -

Try using Picture in Picture instead.

Otherwise I could prepare it in advance for each shot but I'm afraid of a loss of quality by encoding so much. 

There's no need to do that,

This post is not an easy one to answer because there's a few ways of limiting the effect to only the area you want or applying the effect to an entire image & then reducing the size of the image..

@davewire35 Hi.

For now, the idea of ​​using a mask as recommended for the effect could work.

It will work.

But for my VHS type plugins if I use a mask I risk removing information and reducing the extended range of the effect, what I would like is to apply the effect to the scale of my footage.

I'm not entirely sure what that means but it looks like you've used Pan/Crop to reduce the size of the image -

Try using Picture in Picture instead.

Otherwise I could prepare it in advance for each shot but I'm afraid of a loss of quality by encoding so much. 

There's no need to do that,

This post is not an easy one to answer because there's a few ways of limiting the effect to only the area you want or applying the effect to an entire image & then reducing the size of the image..

THANKS :)
I got a bad habit (since sony vegas!) of using the crop to place my images instead of the image in the image but I'm going to do it like that from now on it's much more precise, someone on the forum recommended it to me recently for another topic! Well I did as you told me, picture in picture, bézier mask and it works :) thanks again! Indeed, there must be several solutions but this one does not take too long to put in place, the ideal would just be an option in Vegas to adapt the effect to the size of the footage that we would reduce!