Dans la vitrine, j’ai vu le même modèle avec une meilleure garantie.

Here, la vitrine most naturally means the shop window or the display window/case. So dans la vitrine means something like in the window display.

Depending on context, vitrine can also mean a glass display case inside a shop or museum, but in everyday shopping contexts, English speakers often understand it as the store window.

French often moves a time or place expression to the front of the sentence for emphasis or to set the scene.

So:

• Dans la vitrine, j’ai vu...
• J’ai vu... dans la vitrine

Both are possible.

Starting with Dans la vitrine gives a natural scene-setting effect, a bit like:

• In the window display, I saw...

It does not change the basic meaning very much; it just changes the focus slightly.

J’ai vu is the passé composé of voir and means I saw or I have seen, depending on context.

It is formed with:

• j’ai = I have
• vu = past participle of voir

So:

• voir → vu
• j’ai vu = I saw

French uses the passé composé very often for a completed past action, where English might simply use the simple past.

Because j’ai vu describes a completed event: you saw something at a particular moment.

Je voyais is the imparfait, which is usually used for:

• ongoing past actions
• repeated/habitual actions
• background description

Compare:

• J’ai vu le même modèle = I saw the same model
  → a finished event

• Je voyais souvent ce modèle = I used to see this model often
  → repeated/habitual

So in this sentence, j’ai vu is the natural choice.

Because the verb uses avoir, and with avoir, the past participle usually does not agree with the direct object unless the direct object comes before the verb.

Here:

• j’ai vu le même modèle
• the direct object is le même modèle
• it comes after vu

So vu stays unchanged.

Compare:

• J’ai vu le modèle.
• Le modèle que j’ai vu.

Even in the second sentence, modèle is masculine singular, so vu still looks the same. But if the earlier object were feminine, agreement would appear:

• La voiture que j’ai vue

In French, même usually comes before the noun in this expression:

• le même modèle = the same model
• la même chose = the same thing
• les mêmes problèmes = the same problems

So même here means same, not even.

This is an important distinction:

• le même modèle = the same model
• même le modèle... = even the model...

The article le is also required here because French normally uses an article where English might also use one: the same model → le même modèle.

Because meilleur / meilleure is the normal comparative form of bon / bonne when you mean better.

So:

• bon / bonne = good
• meilleur / meilleure = better

You normally say:

• une meilleure garantie = a better warranty/guarantee

not:

• une plus bonne garantie

This is similar to English using better instead of more good.

Because garantie is a feminine noun.

So the adjective must agree:

• masculine singular: meilleur
• feminine singular: meilleure
• masculine plural: meilleurs
• feminine plural: meilleures

Examples:

• un meilleur prix = a better price
• une meilleure garantie = a better warranty

Most naturally, it describes the model that was seen:

• I saw the same model with a better warranty

In other words, the model on display came with better guarantee/warranty terms.

Grammatically, avec... often adds information to the noun or situation, and here the most natural interpretation is that the same model was being offered with a better warranty.

It can mean either, depending on context.

In everyday product/shopping language, garantie is often best translated as:

• warranty

In other contexts, it can also mean:

• guarantee

So in this sentence, if you are talking about a product model in a shop, warranty is often the most natural English choice.

French usually needs an article before a singular countable noun.

So:

• avec une meilleure garantie = with a better warranty

You normally cannot just say avec meilleure garantie in a standard sentence like this.

The article une shows that it is a better warranty, not necessarily a specifically identified one.