I have two teenage daughters, one being of legal age to obtain a passport and the other is not. My ex and I communicate only through email and I have emailed him several times of a planned vacation overseas next summer requesting a letter of consent for the youngest who is 15. My daughters have followed up by asking him several times to read the email and then outright about the letter I need for the passport in which he ignores them or says "they need to discuss" and then never does. I have given him all the details of the trip but he is not expressing his concerns as to why he will not sign or address this in anyway. Can I file a motion?

With all the stories of one parent taking the children out of the country for "a visit" or "vacation" and then not returning I would be worried too. Turn this around for a second, What would you do if your husband was a foreign national and took the girls for "a vacation" to see Grandma in oh lets say Iran and now you never get to see your children again? Don't get me wrong I am not saying that this is what is going to happen, but I can understand his worries about the possibility. Maybe you need to talk to him in person and explain to him you reasons for taking this vacation now and why you feel it is necessary to take the girls with you. If he is reasonable, maybe he will listen. Then again, maybe he is simply being hard headed?

There is a remedy allowed for this. I too have a similar situation. Rather than requesting via email request via certified US mail and keep track of all attempts. It is important to follow the rules set forth to provide proof that you have tried to obtain consent and that the other party is unwilling.
It doesn't mean that it will be accepted for sure, but it is a place to start.

NOTE: If none of the above documentation is available, the applying parent must submit Form DS-3053 stating why the non-applying parent/guardian's consent cannot be obtained