La vecina de al lado es muy amable.

Because vecina is the feminine singular form of vecino / vecina.

• el vecino = the male neighbour
• la vecina = the female neighbour

The article changes too:

• el
  • masculine noun
• la
  • feminine noun

So this sentence is specifically talking about a female neighbour.

De al lado is a very common expression meaning next door or the one beside us / beside something.

So:

• la vecina de al lado = the neighbour next door

It is not usually translated word-for-word. Even though the pieces look like of the side / from the side, the whole expression works as one unit in Spanish.

You can also use it in other contexts:

• el bar de al lado = the bar next door
• la casa de al lado = the house next door
• el chico de al lado = the boy next door

Because this is the fixed expression al lado, and here you are adding de in front of that whole expression:

• de + al lado = de al lado

Notice that al is already a contraction of a + el.

So the structure is not:

• de + el lado → del lado

Instead, it is:

• de + al lado

That is why del lado would not sound right here if you mean next door.

Because amable is usually treated as a personal quality or characteristic, so Spanish normally uses ser:

• es muy amable = she is very kind / she is very nice

In general:

• ser is often used for more permanent or characteristic descriptions
• estar is often used for states or conditions

So:

• La vecina es muy amable = The neighbour is a kind person / is very nice by nature

You would not normally say está muy amable in this sentence. That would sound unusual and more context-dependent, as if someone is being kind at the moment.

In Spanish, descriptive adjectives often come after ser when you are saying what someone is like:

• es amable
• es simpática
• es alta

So La vecina de al lado es muy amable follows a very normal pattern:

• subject: La vecina de al lado
• verb: es
• description: muy amable

Also, muy means very, so:

• amable = kind / nice / polite
• muy amable = very kind / very nice

Yes. Amable does not change for gender.

So you say:

• el vecino es amable
• la vecina es amable

And in the plural:

• los vecinos son amables
• las vecinas son amables

Many Spanish adjectives ending in -e work like this: they stay the same in masculine and feminine, but add -s in the plural.

If you mean the neighbour next door, la vecina de al lado is the natural and standard expression.

Del lado usually does not mean the same thing in this context. It tends to suggest something more literal like of the side or appears in other structures, not the fixed idea of next door.

So for this meaning, stick with:

• la vecina de al lado

Yes. It is very flexible.

Examples:

• la tienda de al lado = the shop next door
• el piso de al lado = the flat next door
• la habitación de al lado = the room next door
• el restaurante de al lado = the restaurant next door

So it is not only for neighbours. It can describe anything located beside something else.

Yes. If you are talking about a specific neighbour, Spanish normally uses the article:

• La vecina de al lado es muy amable.

This is more natural than just saying Vecina de al lado es muy amable, which would sound incomplete in standard Spanish.

Spanish often uses articles where English might not focus on them in the same way.

You would change the article and noun, and possibly the verb if it becomes plural.

Masculine singular

• El vecino de al lado es muy amable.

Feminine singular

• La vecina de al lado es muy amable.

Masculine plural

• Los vecinos de al lado son muy amables.

Feminine plural

• Las vecinas de al lado son muy amables.

Notice:

• es becomes son in the plural
• amable becomes amables in the plural

In Spain, vecina is pronounced approximately beh-THEE-na in central and northern varieties, because c before i is pronounced like the th in think.

A rough guide for Spain Spanish:

• ve → beh
• ci → thee
• na → na

So: beh-THEE-na

In many parts of Latin America, that c would sound like s, so it would sound more like beh-SEE-na.

It is neutral and very natural. You can use it in everyday speech without sounding especially formal or especially casual.

• amable is a polite, common adjective
• la vecina de al lado is an everyday way to refer to the neighbour next door

So this sentence works well in normal conversation.