I find another bit of Lewis correspondence very helpful just a few letters after the one below on heaven - here he talks about vocation, with some penetrating wisdom for Stacey and me as we find ourselves constantly thinking about our imminent post-studies future.
Do you think one's vocation which looks so cryptic as a whole, is usually fairly clear from day to day and moment to moment? One usually has an idea of what to do next. Need one know any more? It would be a pity if when He came He found me thinking about my vocation at a moment when I would have been better employed writing a letter, making a bed, entertaining a bore - or something quite dull and obvious.
--Collected Letters of C. S. Lewis, 3:781