For the most part if you speak a dialect in and around the two major urban centres (Hanoi and Saigon aka HCMC) then you're good to go. Elsewhere it can get pretty tough.

It's easy to understand urban dialects but hard to understand countryside dialects if you live far away from there. Especially those who come from the north central coast of VN (such as Quảng Bình or Quảng Trị province) they have very different accents as well as dialects, sometimes it sounds like they speak another foreign language not Vietnamese

Im living in North of Vietnam. In the first time I heard the southern accent, I understood only 50%, but 2 month later, it was better then I can understand if they dont speak fast. Almostly, people in all regions can understand Hanoi accent (in North) because it is used in book, radio, television...

It is not easy to understand each other for North and South Vietnamese , I think it is not difficult too. Because we speak Vietnamese, but North and South vietnamese have different accent. I think in the north, Hanoi voice is the easiest to understand for every vietnamese and foreigners, I really like Hanoi voice. In the South, people live in HCM city, and some provinces near HCM city speak easily to understand, too. When one person in the north not Hanoi and one person in the south in the first meeting speak together, maybe they don't understand each other too much. Because they use some local words. But in the second meeting, the third, ... they will understand each other more. Some people in the north speak so fast. I'm Southern, so sometimes I can not hear what my friend who is Northern vietamese says, she speaks so fast and she is talkative. Southern vietnamese speak slower... Your girlfriend is northern right? So I think you can listen what Northern say easier than what Southern say.