I'm wondering if Apple is fine with to the following scenario. I'd like to build an app with premium content based on subscription model. I want to give away unique vouchers to new customers so they can get free access to the app for 3 months. After this period they will be charged regularly.
This would work like:
Perhaps you left out some details - but what you appear to be proposing is an app which is useless to anyone who downloads it, unless they happen to have a voucher from you.
I am not Apple but I have experience in submitting apps and getting them approved, and I think that you are pretty certain to get a useless app rejected. Apple don't want their customers taking the time to download an app and then discovering that they have downloaded nothing.
The following model probably would be approved; but please note that the only certain way to know whether an app will be approved is to build it and submit it.
Provide subscription-based access using Apple's subscription systems.
Provide useful functionality even if the user does not have a voucher and does not subscribe. For example, this might be one month's free access (as opposed to the three months which you give them if they do have a voucher). Or it might be a substantial body of useful permanent reference information related to the field of the app. The key question is "If I download this app without having a voucher, am I wasting my time?" and the answer has to be "No".
To users who do have a voucher, offer a subscription with a three-month free trial period.
The important thing to remember when thinking through all this is that just because an option exists (such as a three-month trial period), that doesn't mean that everybody can access it. You, the application designer, control who has access to what. This means that you can offer in-app purchases to voucher holders which you do not offer to other people.
Edit, giving actual experience:
In an actual app I have successfully done the following:
The reason for "in-app" rather than "subscription" is that at the time subscriptions were introduced, Apple had a policy of allowing subscriptions only for certain categories, such as News, and my app doesn't fall into one of those. Moreover, not using subscriptions has an advantage that people only buy what they want to buy and don't find themselves paying for stuff automatically without meaning to. We therefore don't get bothered by people who delete the app but forget to stop the subscription (remember that there is no way, within an app, of cancelling a subscription, and no way for the publisher to cancel either). So even though the terms of reference for subscriptions are being opened up in the autumn, I'm not at all sure that I'll add subscriptions to the mix.
We do also have a facility for entering a code which a user has as a result of buying one of our other products. The principle here is that Apple quite reasonably don't want us advertising that the user can go off and buy something elsewhere which will then circumvent in-app purchasing, and most particularly they don't want us offering a link that will let the user do that. But actually providing the facility in a suitably discreet way is not in itself objectionable.