Steve Sherman wrote:
I think it's just a bit more complex than that. After all, he was capable of Mazirian before he had published a word.

1) had to learn to write what the pulps wanted to publish (the rejection letters at the Mugar are fascinating in this regard), then

2) had to learn to write that sort of stuff ("Gadget stories") in his own voice (as David says, the improvement in the Ridolphs is instructive).

What I'm not sure of is when the publishers began to realize that they had something special on their hands, if indeed they ever did. By the early 50s it's clear from the correspondence that they have come to regard him as highly reliable, but I suspect they didn't grasp just how original he was for perhaps another decade.
That's why I excluded any part of The Dying Earth. Clearly, JV came to writing with a natural talent. You are correct, when he couldn't sell his fantasy stuff, he turned to SF and had to learn how to write for the market. But, of course, he couldn't entirely suppress his innate abilities and his preferred mode began showing through the pulp fabric as the years progressed. By the early 1950s, an editor like Sam Merwin (or was it his successor, Sam Mines?) could be quoted as saying that there was a recognizable "Vance style" even in the rather crude stories published in Startling Stories and Thrilling Wonder.