Hey for VR its not hugely difficult if your not too scared of C++ their are probably ways in blueprint but i hage no idea how youd go about doing it. What you want to do is modify the scene capture 2d to work for stereo rendering exporting two textures for left and right. Then you just have to set up the portal material as a stereo material by lerping between 0-1 depending on the device being the left or right eye. (Just search stereo material).
Finally as for the camera clipping through the portal plane, use a box with flipped normals instead of a pane and with one of the faces removed. Scale it to 0 along the axis with the open face to turn it into a pane. Now depending on when the vr headset is moving into the portal extend that box until the head is past the teleportation point then teleport when they are deeping inside for a more seemless effect, one teleporting on the same frame set the portal box back to normal.
But theres another problem like hands going through before the head, for this you just create a copy of object and place it their, you could also add some sort of stencil mask to the back of the portal also to prevent objects being seen from the other side while the player is not yet teleported.
Hope this helps.