Answer:
The complete adjective clause is "where we went on vacation last year".
Step-by-step explanation:
The subordinate clause above is an essential relative clause. Relative clauses, also called adjective or adjectival clauses, act just as adjectives as they modify nouns. In this case, the relative clause modifies the noun "cabin". This subordinate clause is considered essential because it is necessary to complete the meaning of the sentence. If it was removed, the meaning of the sentence would be changed since this relative clause helps identify the noun in question. The speaker is talking about the cabin he went on vacation, not any other cabin (see 1).
1) The cabin was perfect