eitel
vain
Being overly concerned with one's own appearance and the impression one makes on others; self-satisfied.
Er ist so eitel, dass er vor jedem Spiegel stehen bleibt.
He is so vain that he stops in front of every mirror.

vain
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vain
Being overly concerned with one's own appearance and the impression one makes on others; self-satisfied.
Er ist so eitel, dass er vor jedem Spiegel stehen bleibt.
He is so vain that he stops in front of every mirror.

The meaning of 'eitel' depends strongly on the context. If it describes a person, it means 'self-satisfied' or 'vain'. If it appears in the fixed expression 'eitel Sonnenschein' (nothing but sunshine), it means 'pure' or 'nothing but'.
Mein Kollege ist ziemlich eitel und achtet immer perfekt auf seine Kleidung.
My colleague is quite vain and always pays perfect attention to his clothes.
As an adjective, 'eitel' (vain) can stand before a noun and is then declined, or it can stand alone after a verb like 'sein' (to be), predicatively. It always describes a characteristic of a person.
Sie ist nicht nur intelligent, sondern auch ein bisschen eitel.
She is not only intelligent, but also a little vain.
The expression 'eitel Sonnenschein' (nothing but sunshine) describes a perfect, problem-free situation. It is often negated ('nicht nur eitel Sonnenschein' — not just sunshine and roses) to express that there are also difficulties.
Eine gute Ehe besteht nicht nur aus eitel Sonnenschein.
A good marriage is not just sunshine and roses.
only
'nur' and 'eitel' can both mean 'nothing but' or 'pure' in one special case. 'nur' is the normal word for limiting something: it is about not more than one thing, person, or amount. 'eitel' has this meaning almost only in the fixed expression 'eitel Sonnenschein' (nothing but sunshine). This means a completely good, harmonious mood without problems. Outside this expression, 'eitel' usually means something else, namely being too concerned with one's own appearance. That is why you can use 'nur' almost everywhere, but 'eitel' in this meaning only in a very limited way.

Nach der Einigung war im Team wieder eitel Sonnenschein.
After the agreement, everything in the team was harmonious again.
Ich habe nur zehn Euro dabei.
I only have ten euros with me.