Option D: When nouns are used as adjectives, then it remains in singular form. In the given sentence, 'two-week' functions as an adjective as it modifies or describes the noun 'vacation'. Hence option D is correct.

Option B: For the same reason, 'two-weeks' is incorrect since adjectives do not have plural form.

Option A: 'Two weeks' does not function as an adjective because it's a compound noun. But, the correct part of speech should be an adjective because the 'vacation' is modified here.

Option C: Two week's is incorrect because the word 'week' is singular whereas the number is plural: two. The correct form should be: two weeks'.