From what I gather from an answer in another sentence, 이다 can be contracted to 다 when it comes after a vowel.
EDIT: Found the other sentence discussion. I copy paste Avanade's answer here below.
Yes! :D It is official as long as you use it for proper occasions. I mean it is possible to use this shortened version when concerned noun words end with a vowel such as 바다(=sea), ex) 나는 가수이다 = 나는 가수다 (casual, shortened) 나는 변호사이다 = 나는 변호사다 (casual, shortened) 우리는 살인자이다= 우리는 살인자다 (casual, shortened) 살인자 = murderer In a conversational situation, many Koreans do not strictly distinguish between singular and plural. If you are sitting for a test, you may need to be a little more strict. 우리는 살인자들이다 살인자들 does not end with a vowel so you can not shorten. 우리는 살인자들다 (X) 가수 = singer 변호사 = lawyer However, 그것은 꿈이다(0) 그것은 꿈다 (x) since word 꿈 does not end with a vowel but a consonant (ㅁ) 꿈 = ㄲ + ㅜ + ㅁ 저것은 산이다(O) 저것은 산다(X) since word 산 does not end with a vowel but a consonant (ㄴ) 산 = ㅅ + ㅏ + ㄴ