Cell-site simulators (CSS)—also known as IMSI Catchers and Stingrays—are a tool that law enforcement and governments use to track the location of phones, intercept or disrupt communications, spy on foreign governments, or even install malware. Cell-site simulators are also used by criminals to send...
Your IMEI, your carrier IP, your packet timing, any DNS your phone leaks, the IP of your VPN endpoint, your transmitter chipset, your likely OS kernel, any unreleased zero-days known to them (and maybe an exploit for them), and also a way to ack TCP packets it never intends to forward in order to sever your connection while letting your device keep taking for as long as possible, which might buy them a little extra time before you realize they’ve captured your session and cut you off.
Everything you said is true, but that is a reduced surface area versus the scenario where you’re sending your traffic naked over the wire. Including your voice traffic. Using a VPN while attached to a stingray is strictly a smaller risk surface.