Why does it say "Ka'asher Tidor
Ibn Ezra: Just like I commanded you to guard your words in Beis Elokim, so you must guard everywhere and at all times that you mention Hashem's name or vow. Guard what leaves your lips!
Rashbam: When you vow to Him at a time of your Tefilah. It is the custom to vow at the time of Tefilah.
Rid: Be careful at the time that you vow, and also afterwards, lest "Te'acher Leshalmo."
Seforno: Just like one who increases words in Tefilah, he thinks to find grace via them, and the opposite occurs, so happens with vows. The Noder thinks to find grace via them, even though he will delay until he will see what Elokim will do to him. The opposite occurs!
Metzudas David: This is a vow to give Tzedakah.
Why does it say "Ein Chefetz ba'Kesilim"?
Rashi: Hashem does not desire Resha'im who vow and do not fulfill.
Ibn Ezra: Do not be a Kesil, for Hashem does not desire them.
Ri Kara: Hashem does not desire those who delay their vows, even though they fulfill them later.
Rid: Hashem does not desire their words.
Seforno: Kesilim think that a vow to Hashem is like a bribe to a mortal king, when it is on condition that the king do [the giver's] desire, and with this he will bestow the bribe. This is Sheker! A vow to Hashem is only submissiveness, and not reward. Therefore, when he does not pay before getting his desire, it is as if he retracts his submissiveness if he will not get it.
Metzudas David: Desire does not last in Kesilim. Sometimes he wants a matter, and afterwards he does not want it. The Noder is considered a fool. It is better to give Tzedakah without a vow!
What is the Chidush that one should fulfill his vow?
Rav Sadya Gaon: Fulfill it quickly.
Ri Kara: Fulfill it immediately. Metzudas David - if not, over time you might cease to desire it, and you will not fulfill it.
Seforno: Fulfill it before you get your desire (refer to 5:3:2:5).
