You will be allowed to check in with just the voucher. However, the key is whether or not the discount gets applied right then and there. Some Cast Members will not apply that discount and will make you come back with the valid Annual Pass. But that is not big deal either. Once you show that valid pass, the discount will be applied for the entire stay.
So, check in, get your room key (your room should be ready by that time anyway), leave your stuff, and go off for your fun. When you return at 4:00 AM, go to your room for sleep. If the discount has been applied, you don't have to do anything more. If it hasn't, just go to the front desk anytime the next day with the valid pass. Or really, any time during your stay.
BTW - Ranchos ain't that bad.