This answer explains why acetic acid does not give a positive iodoform test.
Does acetamide respond positively to the test?
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From wikipedia article of haloform reaction:
Acetyl chloride and acetamide don't give this (iodoform or in general haloform) test.
Why?
Acetamide is not a ketone which is substantially more acidic than the amide. It's about a million times too weak an acid at the alpha protons. Hydroxide is too weak a base. (Quora)
answered May 11 '17 at 7:20
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Acetamide does not give positive Iodoform test. The reason is that if we recall the mechanism of reaction , after incorporation of three halogen atoms on the $\alpha$ carbon of carbonyl carbon, there is a step where the OH- attacks at the carbonyl carbon and $\alpha$ CX3 - leaves. But in acetamide there is a resonance of NH2 group with Carbonyl carbon. So the electrophilicity of Carbonyl carbon decreases and it becomes difficult to attack the less electrophilic carbon by external nucleophile because the overlap of Carbonyl carbon with N of NH2 is very strong (2p $\pi$ - 2p$\pi$ overlap) .so the iodoform reaction is very difficult to occur for acetamide.
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$\begingroup$ The lack of a haloform reaction does not preclude acetamide from reacting with iodine/aqueous base to give a Hofmann rearrangement. $\endgroup$ – user55119 Feb 4 '18 at 16:11
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$\begingroup$ But the question was asked about positive response in haloform test. $\endgroup$ – Soumik Das Feb 5 '18 at 2:22
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$\begingroup$ But the question was asked about positive response in haloform test. $\endgroup$ – Soumik Das Feb 5 '18 at 2:22
