Context: Software Vulnerability Assessment (SVA) plays a vital role in evaluating and ranking vulnerabilities in software systems to ensure their security and reliability. Objective: Although Large Language Models (LLMs) have recently shown remarkable potential in SVA, they still face two major limitations. First, most LLMs are trained on general-purpose corpora and thus lack domain-specific knowledge essential for effective SVA. Second, they tend to rely on shallow pattern matching instead of deep contextual reasoning, making it challenging to fully comprehend complex code semantics and their security implications. Method: To alleviate these limitations, we propose a novel framework ReVul-CoT that integrates Retrieval-Augmented Generation (RAG) with Chain-of-Thought (COT) prompting. In ReVul-CoT, the RAG module dynamically retrieves contextually relevant information from a constructed local knowledge base that consolidates vulnerability data from authoritative sources (such as NVD and CWE), along with corresponding code snippets and descriptive information. Building on DeepSeek-V3.1, CoT prompting guides the LLM to perform step-by-step reasoning over exploitability, impact scope, and related factors Results: We evaluate ReVul-CoT on a dataset of 12,070 vulnerabilities. Experimental results show that ReVul-CoT outperforms state-of-the-art SVA baselines by 16.50%-42.26% in terms of MCC, and outperforms the best baseline by 10.43%, 15.86%, and 16.50% in Accuracy, F1-score, and MCC, respectively. Our ablation studies further validate the contributions of considering dynamic retrieval, knowledge integration, and CoT-based reasoning. Conclusion: Our results demonstrate that combining RAG with CoT prompting significantly enhances LLM-based SVA and points out promising directions for future research.