Yes, Vivo X21 does have the support for the Project Treble but does not offer seamless system updates.
Yes, Vivo X21 does have the portrait mode in the front camera. In fact, both the cameras - front as well rear - supports the portrait mode.
Yes, you can change the fonts in Vivo X21.
Yes, Vivo X21 does have the 3.5 mm headphone jack. It is located on the top panel of the phone.
Vivo X21 is an all-rounder with good performance, decent camera performance and a snazzy new in-display fingerprint scanning technology at the heart of the device.
For the most part, the X21 is a worthy mid-tier device.