The reason I ask is that as you can see in the picture below, I'm standing on the portal blocks and they're doing nothing.

I also can't break them or interact with them in any way (in Survival mode). I can break them in Creative mode, and then it portals me to the next dimension when i jump into the resulting void.

VerticalDimensionStack-1.12.2-0.1.4
CD4017BE_lib-1.12.2-6.4.6
forge-1.12.2-14.23.5.2838-installer-win

Yes the portals only teleport you when they're open, see documenation.

The reason you were unable to break them in survival is probably because there is an unbreakable block like bedrock on the other side.
I see bedrock blocks in your screenshot so you probably forgot to configure the dimension settings so that the mod prevents the generation of bedrock blocks in these dimensions.

Was wondering if the issue was bedrock. I have this in the config:
!prevent bedrock from being generated in ...
rem_block_gen:0 = "minecraft:bedrock"; !the Overworld
rem_block_gen:-1 = "minecraft:bedrock"; !the Nether

I'm guessing one of the other mods is spawning in bedrock after your mod removes it ? Been trying to figure out which mod is doing it, but no luck so far.

Turns out to be Realistic Terrain Gen. I turned the bedrock depth down to 1 block (the minimum) and I get this:

So there are spots where you can break through and ones you can't. Not ideal, but workable.

I would have thought that you would get ported by standing on the portal blocks........

By the way there is also a second method of removing bedrock that runs a bit later in the world generation procedure. Maybe that can kill the late generated bedrock from Realistic Terrain Gen.