Should Vex be played as a pure assassin or as a more measured anti-entry punishment mage?

The most reliable build on Vex is the one that preserves the first spike while still keeping an escape valve. If your item order prevents you from playing the champion's first real window, the build may look strong on paper but it is not reliable.

When should you truly adapt Vex's build instead of simply pushing the standard anti-dash burst route?

You should adapt as soon as the enemy draft stops you from playing the timing the build was meant to serve. If you need to survive longer, absorb more poke, or answer a specific engage pattern, the purchase order must move before you even think about the final six-slot build.

Which enchant should Vex choose when the real issue is mainly surviving the response after a reset?

protobelt is often the most stable choice because it directly supports this game plan: create picks and chain resets.

Vex Wild Rift Best Build Guide (Patch 7.2a)

Vex Wild Rift Best Build Guide

Brutal anti-entry mage build: Vex does not win only when she bursts, but when she turns enemy impatience into a very bad trade. The build must therefore keep dash punishment, burst reliability, and enough margin to survive the answer together.