Due to the game being tagged as "Adult Only" on Steam, it does not appear in some countries, such as Germany.
So, I am not 100% sure if you can activate the Steam key that comes with this due to your location.
In saying that, the version that is here on itch is exactly the same version as what is on Steam and they both receive the exact same updates at the same time.
So if you were to purchase this and the key didn't work, you still have access to iVIBRATE and it will still have all the exact same content that is in the Steam version. The only difference being is that the Itch version would not have Steam achievements and you would have to manually update through itch, either by redownloading it or you can use the Itch app which I believe can auto update like Steam.
Otherwise if it its just for the convenience of having all your games in one place, in your Steam library, if you click "add a game" in the bottom left and select "add a non-Steam game", you can direct Steam to the iVIBRATE Ultimate Edition.exe and then you will have iVIBRATE in your Steam library.