User Interface (UI) testing is crucial for quality assurance of industrial mobile applications, and yet it remains labor-intensive and challenging to automate effectively. Recent advances in Vision-Language Models (VLMs) present a promising solution for automating GUI testing by mapping natural language instructions to pixel-level actions, significantly reducing the manual effort required for writing test scripts and even designing test cases. While numerous VLMs have been proposed and evaluated for GUI testing, they often fail to meet two critical industrial requirements: (1) effectiveness when handling complex, multi-step workflows in industrial applications, and (2) efficiency for large-scale, high-frequency testing environments typical in industrial settings. Toward addressing the preceding industrial requirements, in this paper, we report our experiences in developing and deploying RePeek, a novel approach employing a unified three-stage pipeline for both training and inference, enables a VLM to explicitly detect and reason over discrete GUI elements, thereby overcoming the limitations of pixel-based reasoning for both efficiency and effectiveness improvements. In the first stage, RePeek integrates a lightweight UI-element detector named OmniParser to decompose UI screenshots into a structured element list. In the second stage, RePeek adopts the vision encoder of the VLM to generate the embedding for each element. In the third stage, RePeek fuses these element embeddings with the textual instruction to reason and perform classification directly on the UI elements, empowering efficient small models to achieve superior performance against expensive large models. Comprehensive evaluations on public benchmarks and deployment at WeChat show that RePeek consistently achieves superior accuracy and efficiency compared to state-of-the-art VLMs. Specifically, RePeek enables a fine-tuned Qwen2.5-VL-3B model to outperform a 72B model with 75% less training data, validating the effectiveness of incorporating domain knowledge into VLM-based GUI testing. We conclude by summarizing three key lessons from developing and deploying RePeek, offering insights for both researchers and practitioners working on industrial-strength UI testing.