So far I've only tested Rapid Resizer's virtual reality mode with: (not affiliate links)

• I Am Cardboard Pocket Mini 360 VR Viewer
• DSCVR Headset

Any Google Cardboard style viewer that attaches to your smartphone will probably work. I haven't tested any serious VR headsets yet (Oculus, HTC, etc.) so I don't know if they'll work.

From the step where you enter your design's size, click 3D View.

From the 3D Viewer, tap the VR button.

Attach your VR headset to your phone.

To exit VR:

• On an iPhone, rotate your phone to vertical and tap the X near the upper left.
• On Android phones, tap the X near the upper left.