Midrasch zu Dewarim 23:22

3) Ben Azzai says: What is the intent of "it" (above)? Because it is written (Devarim 23:22): "If you make a vow to the L–rd your G d, you shall not delay to pay it," I might think that if he does delay his vow he is in transgression of "It shall not be accepted"; it is, therefore, written: "It (the offering in our instance) shall not be accepted, but one who delays his vow is not in transgression of "It shall not be accepted." Others say: (It is written (here): "He shall not bethink himself." It (the offering) becomes unfit by thinking (to eat it on the third day), but it does not become unfit by delaying (its sacrifice) beyond three festivals.

3) I might think that if one festival passed and he did not bring them, he transgresses (Devarim 23:22) "You shall not delay (to pay it.") It is, therefore, written (Bamidbar 29:29) "These shall you offer to the L–rd on your festivals." He does not transgress "You shall not delay" until all the (three) festivals of the year have passed. R. Shimon says: Three consecutive festivals, with the festival of matzoth first.

3) If "the children of Israel," I might think, only the children of Israel. Whence do I derive for inclusion proselytes and bondsmen? From "and say to them." I might think that (also) excluded is a minor (from twelve years and a day) who is able to utter (a vow distinctly). It is, therefore, written "if he utter a vow." It is written here "a vow," and elsewhere (Devarim 23:22) "a vow." Just as "a vow" here is subject to "if he utter," so, "a vow" there is subject to "if he utter." And just as "a vow" there is subject to "do not delay (to pay it"), so, "a vow" here is subject to "do not delay."