Replacing "libical1" with "libical1 | libical1a" in the DEBIAN/control file of cairo-dock-plug-ins_3.4.99.alpha1.1-20150103-git3269-7dde429-0ubuntu1-ppa0-vivid_amd64 fixed this issue for me.
Probably it would be a good idea to fix this issue upstream as well.
Replacing "libical1" with "libical1 | libical1a" in the DEBIAN/control file of cairo-dock- plug-ins_ 3.4.99. alpha1. 1-20150103- git3269- 7dde429- 0ubuntu1- ppa0-vivid_ amd64 fixed this issue for me.
Probably it would be a good idea to fix this issue upstream as well.