Can I suggest a wacky idea?
When the community manager takes a 2-week vacation, don't find a replacement.
Or, at most, find someone to remove any spam/offensive material and let the rest be.
Track your data to watch what happens.
Is there a significant dip in growth or activity?
Are there upset members threatening to start their own community?
Do you have a full-scale mutiny on your hands?
2 weeks might not be a long enough time-line to show what difference a community manager makes, but it should give you some indication.
Don't make this standard practice, but every once in a while it might be interesting to see what difference your community manager makes. It's useful for proving your value.