Which Outlook technology―managed add-ins built with Visual Studio, MAPI, VBA macros, or apps for Office―is best for your particular scenario? It is important to note that all these technologies will be supported in the foreseeable future. However, if you're looking for a new way for users to interact with Office customizations, apps for Office is a great place to start.
Read Selecting an API or technology for developing solutions for Outlook 2013 for an in-depth look at your choices.
VBA code is often stored and distributed directly within a document. While this makes initial deployment and distribution easy, it also makes updating code in existing documents more difficult.
For developers who want to leverage more extensive customizability of Office, or who want to target Office 2007 or Office 2010 (apps are available starting in Office 2013), VSTO is the primary option.