Vivo X21 looks similar to Vivo's V-series flagship, Vivo V9. Phone features an all-black aluminum frame and a glass back with tapered edges, which make it very comfortable to hold. It's about 7.5 millimeters at its thickest point, but it's surprisingly easy to handle, especially considering large 6.28-inch screen. It has a glass and metal design, which surely is solid to hold. It is densely built and glass back feels strong too. There's no doubt that phone looks like Vivo V9, but there's no fingerprint scanner to be found on back. To be honest, if Vivo would've named it V9 Pro, that would've made no difference.
Front of device is interesting, as it was on Vivo V9. Most of it is covered with edge to edge display, which has a notch. Notch holds earpiece and sensors, together with front facing camera. There's a slight chin at bottom and minimal bezels at sides, but rest of front is all covered with screen.
At back of device, you'll find a dual camera setup vertically placed on top right with an LED flash just below. It's quite small compared to other vertically placed sensors. There's a sharp, thin rim around camera, but it's quite subtle and shouldn't get scratched when placed on rough surface Vivo provides a plastic case to protect body anyways. Back is covered with glass, giving a premium feel and look to it. It shines well, but not as much as Mirror black finish on OnePlus 6. You'll see some Vivo branding in middle of phone and some tiny writing on bottom part, with rest of back kept clean and simple.
One thing that many people will notice is that there's no fingerprint scanner anywhere to be seen on this phone, and this because Vivo X21 comes with an in-screen fingerprint scanner, first in any smartphone.
A welcome design decision is inclusion of 3.5 mm headphone jack which is now found on top of phone. On right side of phone, you'll find a power button with volume keys with left side kept clean. At bottom, there's a single bottom-firing speaker, sim tray and a Micro-USB port, which is a bummer as most phones now come with USB Type-C instead, especially at this price.
All-in-all, overall look and feel of the phone isn't bad for the price. If you look at the OnePlus 6, there's a lot of similarity between the two and even the quality isn't compromised. However, this design is repetitive and most people will be expecting something new in design department of Vivo X21.