Can Pandemic Legacy be played as "vanilla" Pandemic?
In fact, the Legacy rules themselves suggest playing a few games if you aren't familiar with Pandemic.

IF YOU'VE NEVER PLAYED PANDEMIC...
We recommend that you play a few games without any of the special
&quot;Legacy&quot; rules to get a feel for the decisions you will have to make.
Read this rulebook then play without using the following rules:
•Game Months, Legacy Deck, Dossiers
•Objectives, Funding, Game Calendar, Panic Level
•Scars and Lost Characters
•Game End Upgrades
The object of these warm-up games is to cure four diseases before any of
the game-losing conditions are triggered (noted on page 14 of these rules).
You should play at least two games this way to get a feel for the game before
adding the additional rules.