I have been asked to quality review two code bases. I've never done anything like that, and need advice on how to perform it and report it.

Background

There are two providers of code, one in VB and one in C (ISO 9899:1999 (C99)). These two programs do not work so well together, and of course, the two suppliers blames each other. I will therefore as a independent person review both codes, on a comprehensive level review the quality of the codes to find out where it is most likely that the problem lies. I will not try to find problems, but simply review the quality and how simple it is to manage and understand the code.

Edit: I have yet not received much information about what the problem consists of. I've just been told that I will examine the code in terms of quality. Not so much more. I do not know the background to why they took this decision.