[dumbfoundded]

Your boss in the wrong for asking you. At the same time, you will probably be viewed negatively for not doing it. Your call.

[quirkot]

A potential reply to avoid coming across poorly could be something like: "what time code or project number should I report my time under when I stop my vacation and go on-call?"

[bouchardm]

I find this is a great response, unfortunately we are a SASS, only one project number :/