Yes, Vivo X21 does have the Smart Split function.
Yes, Vivo X21 does have the 3.5 mm headphone jack. It is located on the top panel of the phone.
No. Vivo X21 has MicroUSB. It does not have the USB Type-C port.
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.
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.