Hello Linux Geeksters. As you may know, Ubuntu 15.04 Vivid Vervet and all the flavors are scheduled for release on the 23rd of April, 2015.
Unlike rolling releases (like Arch Linux), Canonical does not upgrade the kernel after launch, on regular Ubuntu releases, so they have to choose a suitable one from the beginning. Usually, the Ubuntu team uses the latest stable kernel that gets released before the kernel freeze.
Most likely, Ubuntu 15.04 will be using Kernel 3.19, which should be released next week.
For now, Ubuntu 15.04 Vivid Vervet uses Kernel 3.18.4 on the stable branch, while the unstable branch is based on Kernel 3.19 RC7.