Better late than never
Here’s some good news if you own a V9 or X21: vivo is currently rolling out Android Pie for both 2018 phones, and this is one of the rare instances that the brand is rolling out the update via OTA. Previously, V5, V5 Plus, and V5s owners had to bring their phones to the nearest service center to get upgraded to Android Nougat.
According to the changelog for the X21, the update will introduce a new UI, Dark Mode, an updated Game Box, updated Shortcut center, and several under-the-hood tweaks. The update weighs around 3GB, so make sure you have a stable and fast internet connection before updating your phone.