its nice but not expected. Tipping in Europe is NOT like in USA. In Europe servers and wait staff get a proper living wage, they do not rely on tips to pay the rent like they do in USA.
In Europe I generally just round up a couple of Euros. For example if the bill is €22.50 I would give €25. You will not get the "suggested" tip amount on the bill like you do in USA.
I have never used the meal plan vouchers, but again I would probably just leave a couple of Euros cash on the table at the end.