First of all, as you thought, いいになる isn't correct, it should be よくなる. だろう can be used for assumptions, suspition, guess, etc, and it includes the guess for the future, for something uncertain. Its formal form is でしょう. However, there are several translations for "it will be good" depending on the context, for example, with guess, 良いでしょう, 良いことでしょう, 良くなるでしょう, and without guess, 良いです, 良いことです, 良くなります and more. In the case of "it will be good to have a vacation", it would be 休みをとることは良いことです / 良いことでしょう for the construction of this sentence though it's not a literal translation because it would sound unnatural. I hope this was helpful.

I think the translation is "良いと思う" or "良いね". "it will be good to have a vacation" I translate it into, for example, "休暇をとるのは良いね", "休暇をとるのが良いと思う" etc.

The expression you want use should be "(nounが)よくなる" , "(nounが)絶対によくなる", or "いい+nounになる", "絶対にいい+nounになる". And the sentence "It will be good to have a vacation" can be translated like "休みがあればいいと思う", but the meaning of the translation has been changed a little. It means "I think it will be good if I have a vacation".

What about, 休暇をとるのは良い事になるでしょう。 or it is not really a word for word translation but, 休暇をとれば良い結果となるでしょう。