You own the copyrights. Unless you get an exclusive contract you may post them basically in any site you want. I imagine you enrolled on Amazon's special program otherwise you would be able to post your story on multiple sites. If you want to continue on Amazon's special program I suggest you don't repost your story on Tapas.
If you still want to post your story on Tapas then just leave Amazon's special program and sell your books on amazon kindle without any of the extra benefits of being on Amazon kindle Vella or on their special program. Doing that won't require you to be exclusive to amazon.