I know how panic inducing long flights can be. However, I have never ever seen anybody v* on a flight...and if you were very very unfortunate and this did happen to you, I'm sure that you could politely explain the situation to a member of cabin crew and they could find you somewhere else to sit.

Although really, DO NOT WORRY. The chances of this happening are so slim that it needn't cross your mind. Anyway, if somebody was to get sick it would most probably be due to travel sickness, so nothing contagious.

Just look forward to your holiday and don't worry about the flight...have fun!!