Why does the Torah see fit to add "Motzei Sefasecha Tishmor ... "?
Rashi: To add an Asei to the Lo Ta'aseh. 1
Ramban (on Pasuk 23): Refer to 23:24:1:1. In any event, having made the Neder, make sure to keep everything that you promised - and do not think that you will receive reward for your good intentions when making the Neder, even if you don't manage to carry it out. 2
Rosh Hashanah 6a: We learn a Lo Sa'aseh from "Tishmor", and the Asei from "Motzei Sefasecha." Tosfos (DH Zu)
What is the difference between "Neder" and "Nedavah" (in the realm of Hekdesh)?
Kinim 1:1: "Neder" is where the person says 'Harei alai ... ', where he makes a personal undertaking to bring a Korban; "Nedavah", where he says 'Harei Zu ... ', declaring the animal Hekdesh. 1
Kinim 1:1: A Neder has Achrayus (if the animal that he subsequently chooses gets lost or dies, he is obligated to replace it), but a Nedavah does not.
Seeing as the Torah is discussing a Neder, why does it add the word "Nedavah"?
Torah Temimah (citing Zevachim, 2b): If the Neder is slaughtered Lishmah, he fulfilled his Neder; but if it is slaughtered 'she'Lo li'Shmah' (for the sake of a different kind of Korban) then it is brought as a Nedavah - and the owner is obligated to bring another one to fulfill his Neder.