I'd never be without a vane, and all of the comments above I'd agree with, especially about sail trim. We had a Monitor (excellent gear
and great firm to deal with) on our last boat which was a fast fin and spade Frers design and never had an autopilot at all.
But there are times when the vane doesn't work so well - close to land where there are lots of wind effects from the shore, or squally conditions, for example, when you can be up and down every five minutes to re-trim the vane. Some people find this frustrating at first, but stick at it and you'll get to know the gear
really well, and know how to set the boat up to reduce the nuisance factor - or stick the autopilot on.
We have a vane (Windpilot) on our current
boat, together with a powerful and expensive autopilot. But the autopilot has already quit once within a few thousand miles, and I cannot even imagine how we'd break the vane.....
Best of luck with the Monitor.