Okay final verdict, because my previous idea on how to address it was needlessly complex. Here's the nerfing of the spell as it will appear in the upcoming patch notes:
The Open Dimensional Portal spell (and all Rods/Scrolls/Magic Items that can cast it) have been nerfed, because it was proven to be the single most abusable spell in the game. Previously it had a 50% chance of creating a Vortex in your square. It retains this probability, but crucially, it will only function on 33% of maps. On 67% of maps, the spell will always fail, and the player casting the spell will never be told if the spell failed because of the 50% chance or because it’s 100% impossible on this map. All of these rules are spelled out explicitly in the query of the Spell or any Object that can cast it.