La solicitud de vacaciones está lista.

La is the feminine singular definite article, meaning the.

It is used because solicitud is a feminine singular noun:

• la solicitud = the request / the application

Unlike English, Spanish nouns have grammatical gender, so the article has to match the noun.

Solicitud usually means request, application, or formal request, depending on context.

In this sentence, solicitud de vacaciones means something like:

• holiday request
• vacation request
• request for time off

It sounds a bit more formal than just petición.

De vacaciones means for vacation/holiday or related to time off here.

So:

• solicitud de vacaciones = vacation request / holiday leave request

Spanish often uses noun + de + noun where English might use two nouns together:

• solicitud de empleo = job application
• casa de campo = country house
• tarjeta de crédito = credit card

So this structure is very common.

In Spanish, vacaciones is normally used in the plural, even when English might say vacation in the singular.

So:

• Estoy de vacaciones = I’m on holiday / on vacation
• las vacaciones de verano = the summer holidays

This is completely normal in Spanish. A learner should usually think of vacaciones as a plural noun.

Spanish uses both ser and estar, and here estar is used because lista describes a state or condition: the request is ready.

• está lista = it is ready

If you used es, it would sound wrong here because readiness is not a permanent characteristic.

A useful comparison:

• La comida está lista. = The food is ready.
• Madrid es grande. = Madrid is big.

Because lista is an adjective agreeing with solicitud, which is feminine singular.

Agreement in Spanish means the adjective matches the noun in gender and number:

• la solicitud está lista = feminine singular
• el documento está listo = masculine singular
• las solicitudes están listas = feminine plural

So lista matches solicitud.

No. Lista can mean different things depending on context.

Here, estar listo/a means:

• to be ready

But lista can also be the noun list:

• una lista = a list

And in some contexts, listo/a can mean clever or smart, though that meaning is more often linked with ser:

• Es muy listo. = He’s very clever.

So context matters.

This is the most neutral and natural word order in Spanish:

subject + verb + adjective

• La solicitud de vacaciones = the subject
• está = the verb
• lista = the adjective

Spanish can sometimes change word order for emphasis, but this version is the standard way to say it.

Yes, if the context already makes it clear that you are talking about a vacation request.

• La solicitud está lista. = The request/application is ready.

Adding de vacaciones makes the meaning more specific.

It can match both, depending on the variety of English.

For a learner of Spanish from Spain:

• vacaciones often corresponds naturally to holidays in British English
• in American English, it is often translated as vacation

So solicitud de vacaciones could be understood as:

• holiday request (UK-style)
• vacation request or time-off request (US-style)

If you were talking about more than one request, you would say:

• Las solicitudes de vacaciones están listas.

Changes:

• La → Las
• solicitud → solicitudes
• está → están
• lista → listas

Everything has to agree in number.