Marie s’est fait laver les cheveux avec un shampooing qui sent bon la vanille.

This is the faire + infinitive construction, often called the causative.

se faire + infinitive means to have something done to oneself.

So:

• Marie s’est fait laver les cheveux = Marie had her hair washed
• literally: Marie made herself wash the hair, but that literal wording is not how we translate it naturally

This structure is very common in French:

• Elle s’est fait couper les cheveux = She had her hair cut.
• Je me suis fait examiner par un médecin = I got examined by a doctor.

It often suggests that someone else performed the action for the person concerned.

Because se faire + infinitive does not mean that the subject does the action personally. It means the subject has the action done to them.

So:

• Marie s’est lavé les cheveux = Marie washed her own hair.
• Marie s’est fait laver les cheveux = Marie had her hair washed.

That little s’ marks that the action is happening to Marie herself, even though someone else is probably doing the washing.

In the construction faire + infinitive, the past participle fait normally stays invariable.

So we say:

• Elle s’est fait laver les cheveux
• Elles se sont fait laver les cheveux

not faite or faites here.

This is a point that often surprises learners, because many other past participles with être do agree, but fait in se faire + infinitive usually does not.

French often uses the definite article with body parts when the owner is already clear, especially in reflexive-style constructions.

So:

• Elle s’est lavé les mains = She washed her hands.
• Il s’est cassé la jambe = He broke his leg.
• Marie s’est fait laver les cheveux = Marie had her hair washed.

English prefers her, but French often prefers the in these cases.

The difference is who performed the washing.

• Marie s’est lavé les cheveux = Marie washed her own hair.
• Marie s’est fait laver les cheveux = Marie had her hair washed, probably by someone else.

So the second sentence is what you would expect, for example, at a hair salon.

Yes, that is possible, but it sounds a bit different.

• Marie s’est fait laver les cheveux focuses on Marie receiving the service.
• Marie a fait laver ses cheveux can sound more like Marie arranged for her hair to be washed.

The version with se faire is the most natural one when talking about something done to oneself, especially personal care.

Here avec means using or with.

So:

• avec un shampooing = with a shampoo / using a shampoo

It tells you what product was used during the washing.

This is different from something like par, which would usually introduce the agent in a passive idea:

• par le coiffeur = by the hairdresser

But here the sentence is talking about the product, not the person doing the action.

It is a relative clause describing un shampooing.

So:

• un shampooing qui sent bon la vanille = a shampoo that smells nicely of vanilla / a shampoo with a pleasant vanilla scent

The pronoun qui means that/which and refers back to un shampooing.

So the shampoo is what smells of vanilla, not Marie and not the hair.

Because sentir bon is the normal French expression for to smell good / to smell nice.

• Ça sent bon = That smells nice.
• Cette fleur sent bon = This flower smells lovely.

sentir bien usually does not mean the same thing. It can mean something more like to sense/perceive well, depending on context, and it is not the usual way to say that something has a pleasant smell.

So:

• sent bon la vanille = smells nicely of vanilla

is the idiomatic phrasing.

After sentir bon, French often uses the noun directly:

• Ça sent bon le café
• Ça sent bon le pain chaud
• Ça sent bon la vanille

This is a common idiomatic pattern.

You may also see sentir la vanille, which simply means to smell like vanilla or to smell of vanilla.
But sentir bon la vanille adds the idea that the smell is pleasant.

So:

• sent la vanille = smells of vanilla
• sent bon la vanille = smells pleasantly of vanilla

Qui refers to un shampooing.

So the structure is:

• avec un shampooing
• qui sent bon la vanille

Together:

• with a shampoo that smells nicely of vanilla

A learner might briefly wonder whether qui could refer to les cheveux, but it does not. It refers to the nearest logical noun, shampooing.

Yes. Laver les cheveux is a completely normal way to say wash the hair / wash someone’s hair.

Examples:

• Le coiffeur lui a lavé les cheveux. = The hairdresser washed her hair.
• Elle s’est lavé les cheveux. = She washed her hair.
• Elle s’est fait laver les cheveux. = She had her hair washed.

French often uses cheveux where English naturally says hair, even though cheveux is grammatically plural.

In normal usage, yes, that is the usual interpretation.

se faire + infinitive strongly suggests that the subject had the action done to them, often by another person.

So most readers will understand:

• Marie did not wash her own hair.
• Someone else washed it for her or on her.

That is why this structure is especially common for salon, medical, or service situations.