Pass contrib module tokens to custom event in rules
When the Rule gets invoked, you can send any number of parameters along with it. Just make sure they match the expected data type.

rules_invoke_event('stripe_subscription_event', $acting_user, $plan, $stripe_subscription_event);


This will trigger the Rule and send the variables along. It is up to you call this function and obtain these values somehow. 

Since your Rules description is A user subscribed, I suppose you should the submit function of that event to invoke the Rule. You might be able to grab the values from the $form_states['values'] array. I have no idea what the Stripe subscription module does, so I am not sure how you should obtain these values.

I don't think you can use tokens. Those are specifically for rendering content and I have never seen them used in code. (You might obtain the values by copying some of the logic though.)