Vết bẩn này khá nhỏ.

Vết bẩn is a noun phrase meaning stain, spot, or dirty mark.

• vết means a mark, trace, or streak
• bẩn means dirty

So vết bẩn is literally something like a dirty mark.

Here, vết is not just a generic classifier. It is the main noun. Vietnamese often builds noun phrases this way, with the main noun first and the describing word after it.

In Vietnamese, demonstratives like this and that usually come after the noun.

So:

• vết bẩn này = this stain
• vết bẩn đó / kia = that stain

This is a very common Vietnamese pattern:

• cuốn sách này = this book
• cái bàn này = this table

So English this stain becomes Vietnamese stain this in word order.

Vietnamese often does not use a word like English is when describing something with an adjective.

So:

• Vết bẩn này khá nhỏ
  literally follows the pattern this stain fairly small

That is a complete natural sentence in Vietnamese.

This is very common:

• Nhà lớn = The house is big
• Cô ấy đẹp = She is beautiful

You usually do not add là before an adjective in this kind of sentence.

Khá means fairly, rather, or quite.

It shows a moderate degree, stronger than a little, but usually not as strong as very.

A rough scale is:

• hơi nhỏ = a little small
• khá nhỏ = fairly / rather small
• rất nhỏ = very small

So khá gives the sentence a softer, more moderate feeling than rất.

Because words like khá, rất, and hơi usually go before the adjective they modify.

So the pattern is:

• khá nhỏ = fairly small
• rất lớn = very big
• hơi khó = a little difficult

Putting khá after nhỏ would sound wrong.

Nhỏ is the most natural and neutral word here.

For the size of an object, mark, or stain, nhỏ is the normal choice.

Bé can also mean small, but it is more often used for:

• children
• cute things
• informal speech
• things being described in a softer or more affectionate way

So for a stain, nhỏ sounds more natural than bé.

Yes, you can.

Adding cái is possible, especially in everyday speech, but it is not required here.

• Vết bẩn này khá nhỏ = natural
• Cái vết bẩn này khá nhỏ = also natural, with a slightly more conversational or pointed feel

However, since vết bẩn is already clear, speakers often leave cái out.

It is most naturally understood as singular here: this stain.

Vietnamese does not always mark singular and plural as clearly as English does. Context often tells you.

If you wanted to make it clearly plural, you could say:

• những vết bẩn này = these stains

So này by itself does not make something plural. It just means this/these, depending on context.

A rough Northern-style pronunciation is:

• vết ≈ /vet˦˥/
• bẩn ≈ /ɓə̆n˧˩˧/
• này ≈ /naj˨˩/
• khá ≈ /xa˦˥/
• nhỏ ≈ /ɲɔ˧˩˧/

A few useful pronunciation notes:

• vết has a sắc tone: rising
• bẩn has a hỏi tone
• này has a huyền tone: low/falling
• khá has a sắc tone
• nhỏ has a hỏi tone
• kh is not like English k; it sounds more like the ch in Scottish loch
• nh sounds like ny in canyon

If IPA is not helpful, focus first on getting the word order and tones roughly right.

Not much.

The natural order is:

• Vết bẩn này = this stain
• khá nhỏ = fairly small

So the full sentence is:

• Vết bẩn này khá nhỏ

You should not move này before the noun, and you should not move khá after nhỏ.

For example, these are not natural:

• này vết bẩn
• nhỏ khá

So this sentence is a good basic pattern to remember:

noun phrase + degree word + adjective