how can a minhag get the status of a neder when nedarim have so many rules such as tnai kafel etc so where and how was the neder made?

The issue of "tnai Kaful" applies to the validity of a "tnai" a condition, not to a neder. However, in regard to a neder, The Shulchan Aruch (Yoreh De'ah 114) says that when somebody knowingly takes upon himself not to do something which is permitted according to Halacha, he is obliged to keep to his undertaking unless he does a "Hatarat nedarim."