I've been reading/skimming through Daniel Pool's book "What Jane Austen Ate and Charles Dickens Knew" and he also outlines the distinction between a vicar and a rector.

There were two classes of livings that could be given. Collations were awarded by bishops, and the recipients were called rectors, and they were entitled to all the tithes paid to the church by the local parishoners. Vicars, on the other hand, were not entitled to all the tithes, (because they were technically only representatives of the 'real' rectors). Usually, says Pool, vicars would receive part of the produce given to the church, while the rector of such a parish would get all the corn, wood and hay.

Then he goes on to say that usually such parishes had long ago been purchased by some monastic order or other, which became the de facto "rector", and that later many of these parishes became part of the patronage of large landowners.

These definitions are kind of unclear to me, since by this definition Mr.Collins would be a vicar, but in the text JA has him refer to himself as the rector. But, he also talks about negotiating the tithes, which does make it sound as though they are not automatically due to him through the living.

I'm not sure how well this adds to the distinction we're discussion here...I think I'm more confused than I was when I started writing this post! :)