Learn German: Wir brauchen eine Bekannte vom Chef. - We need an acquaintance f of the boss.

Bekannte is a nominalized adjective. By capitalizing the adjective bekannt (known), it turns into a noun meaning a known person or an acquaintance. In German, all nouns are capitalized, even those created from adjectives.

Since Bekannte is a noun now, does it have a fixed ending?

No, and this is the tricky part: it changes endings exactly like an adjective! Because the sentence refers to a female acquaintance in the accusative case, it takes the feminine ending -e after the article eine. If you were talking about a male acquaintance, you would use the masculine accusative ending, making it einen Bekannten.

If I want to say the acquaintance instead of an acquaintance, does the word change?

For a female acquaintance in this sentence, it stays the same: die Bekannte. But remember that because it follows adjective rules, the ending depends on the article before it. For a male acquaintance, it would be einen Bekannten (a male acquaintance) or den Bekannten (the male acquaintance).

Why does the sentence use vom Chef instead of the genitive case des Chefs?

Using von + dative (like vom, which is von + dem) is a very common way to show possession or relationship in everyday spoken German. Saying eine Bekannte des Chefs (using the genitive) means exactly the same thing and is perfectly correct, just a bit more formal.