The research described in this paper is concerned with the application of information retrieval to software maintenance, and in particular to the problem of recovering traceability links between the source code of a system and its free text documentation. We introduce a method based on the general idea of vector space information retrieval and apply it in two case studies to trace C++ source code onto manual pages and Java code onto functional requirements. The case studies discussed in this paper replicate the studies presented in references [3] and [2], respectively, where a probabilistic information retrieval model was applied. We compare the results of vector space and probabilistic models and formulate hypotheses to explain the differences.