While I usually travel to North Vietnam, I did travel to the southern part when I vacationed in Nha Trang.
One of the dishes that seemed popular there was Hủ Tiếu.
In general, southern Vietnamese food is more sweet and sour while northern Vietnamese food has more garlic and savory flavor.
I had Hủ Tiếu many times in Nha Trang, and unfortunately I did not really enjoy it. I definitely prefer northern VN cuisine over southern VN cuisine.