Can I use 3G on my iPad 3 in Vietnam?

I'm going to Vietnam in a few weeks for the first time and I want to bring an iPad 3 LTE version with AT&T to use the internet to facebook and email without bringing a laptop.

Will I be able to take out my AT&T data sim and use a Vietnamese mobile company data sim card in the iPad?

I have only been using wifi on the iPad and not activated a data plan with AT&T yet, do I need to do that?

As I understand there is no 4G in VN, but there is 3G available.

Do I need to get the iPad "unlocked" first prior to using it abroad like cellphones? I bought my iPad from BestBuy if that makes any differerence.