Web-measurement studies treat commercial VPNs as interchangeable vantage points within a country, assuming that any VPN in a particular country is as good as any other. We show that this assumption does not hold: the same country measured through different VPN providers yields materially different conclusions about where endpoints sit, who hosts them, and which physical replicas serve them. Using large-scale browser-based measurements across fourteen countries and four major VPN providers, complemented by targeted DNS and replica-selection probes, we examine sources of this variability across three layers of the VPN-to-endpoint path: vantage identity, name resolution, and replica selection. We find that the variability is driven primarily by layers below the client: commercial VPN providers operate their own in-country DNS infrastructure, often intercepting queries regardless of client configuration; CDNs steer on the exit network, sending identical queries to different replicas; and peering paths route identical DNS answers to different physical facilities. We distill these findings into a set of reporting practices for VPN-based Web measurement.

翻译：Web测量研究将商业VPN视为同一国家的可互换观测点，假定特定国家内的任意VPN提供商均可相互替代。我们证明这一假设不成立：通过不同VPN提供商测量同一国家，会对端点位置、托管方以及提供服务的物理副本得出实质性差异的结论。我们基于跨越十四个国家、四家主要VPN提供商的浏览器大规模测量，辅以定向DNS与副本选择探测，从VPN到端点路径的三个层面（观测点身份、名称解析、副本选择）分析这种变异的根源。研究发现，变异性主要由客户端以下的网络层驱动：商业VPN提供商运营自有国内DNS基础设施，常无视客户端配置拦截查询；CDN会根据出口网络进行调度，将相同查询导向不同副本；对等互联路径会将相同DNS应答路由至不同物理设施。我们将这些发现提炼为一套面向基于VPN的Web测量的报告实践规范。