Firstly depending on what version 3.8.x you're running you may or may not be able to actually run your forum on php 5.6. I believe you'd need to be running 3.8.8 for php 5.4, and 3.8.9 for php 5.5. According to the vb faq
http://www.vbulletin.com/en/faq/ 3.8.9 would support php 5.6. If you are running 3.8.7 or earlier you won't be able to upgrade your php version without having to make changes to the core files to make them compatible with the new php version.