1)
What are "Nedarai"?
1.
Rashi: The Korbanos that I vowed.
2)
This is precisely verse 14. Why is it repeated?
1.
Radak: It is to show (via the coming verse) that he will pay his vows in Chatzros Beis Hashem 1 .
2.
Malbim: Here, "Negdah Na l'Chol Amo" means corresponding to His entire nation.
1
At that time, it was permitted to offer on Bamos. David said that he would offer in Chatzros Beis Hashem (refer to 116:19:1:1 - PF).