There is no such thing as a 'plain vanilla' SAP HR implementation. When customers start their SAP implementations it's common to hear project managers and sponsors say that this will be a 'plain vanilla' implementation. Customizations and programming will be kept to a minimum, and modifications will be strictly forbidden. Those are fine intentions, but back in the real world: there is no such thing as a 'plain vanilla' SAP HR implementation.
Why can't we have plain vanilla SAP HR implementations? Because HR policies and processes are implemented differently at each customer – for good, valid business reasons. Companies vary in their recruiting, training, and hiring processes; they have different benefit plans and policies; they have special compensation schemes. Yes, much of all this is the same from one customer to another – there is a base of common policies and practices; however, the remaining differences require thought (i.e. analysis and design) to implement into SAP HR – or any other system.
A more realistic goal for SAP HR implementations is to implement their processes and policies into the standard framework provided by SAP. The system has a flexible and functional framework for accommodating a wide range of customer requirements. Companies run into implementation and maintenance issues when they go outside that framework. Knowing that framework is the job of SAP Education (who will train your company's staff) and whichever implementation consulting team you choose. Though you won't end up with vanilla, you will get your company's flavor in a good SAP HR implementation.