Can I subscribe to historical API without subscribing to Vortex API?

No. The Historical Data API is part of Vortex, so you need to be registered on Vortex to use it. There's no separate, standalone historical-data subscription.

Once you're set up on Vortex, you can use the Historical Data API along with the other APIs — and, like the rest of Vortex, it's free to use.

To get started, see How to register for Trading API through Vortex.