Can you tell me what kind of surface material is coated on the iron NPs?

then, we could assume an event between them.

This article would definitely helpful to you

DOI: 10.1039/C3RA23454A

Please refer one more article.

Thank you OMKAR

Would be afraid that iron nanoparticles get dissolved in acetic acid.

I agree with Dr Sobisch. Iron NPs will be corrosive in the presence of acid unless fully stabilized with a coating material, which may be resulting to an aggregation.  

Thank you Dr Sobish and Kim. The amount of avetic acid is really low (pH of solution is 6.5) and the preparation was carried at 40 C