History
VLESS is the successor to the VMess protocol from the V2Ray (Xray) ecosystem. VMess had a big drawback — a dependency on system time. If your clock drifted a minute from the server's — that's it, no connection. Developers got tired of this, and in 2019-2020 they released VLESS (VMess Less — "less than VMess").
Advantages
The main advantage of VLESS is its invisibility when paired with XTLS/Reality. VLESS wraps your traffic in ordinary TLS. But Reality uses someone else's legitimate site (for example, microsoft.com) as a "mask." The censor sees you connecting to Microsoft and thinks: "Oh, legitimate traffic, let it through."
Recently the protocol was upgraded: it now has built-in post-quantum encryption (ML-KEM) to protect you from future quantum computers.
Disadvantages
On its own, without a TLS wrapper, VLESS is completely open and useless. It's just a skeleton. In late 2025 in Russia they started successfully blocking "pure" VLESS over TCP. It survives only paired with xtls-rprx-vision or HTTP transports.
Summary
VLESS is a tool for professionals. On its own it's zero, but paired with Reality it's one of the most reliable ways today to hide from DPI.