If your portal sides are North - South, only the north bottom block can be used to light the portal
If your portals sides are West - East, only the east bottom block can be used to light the portal

Note: this is when making the portal in a nether portal shape, not when doing the special vethea portal shape

Also, I was able to create a vethea portal in the overworld with mortum blocks by placing them in the special shape, I don't know if this is intended, since I thought the nightmare bed was the only way to access the dimension