Van Gogh can surely never be considered an Impressionist. He's usually thought of as a Post-Impressionist, along with Paul Gaugin, Paul Cezanne and Georges Seurat, but that's a bit of a rag-bag classification because those four (and others) didn't have a close common approach or agenda.
Call me Ishmael...
Van Gogh knew many of the impressionistic painters like Lautrec and he did live in Paris in the 1880's for a while.
IMO, its hard to classify him for he isolated himself for most of the time when he painted.
When you read his letters, he always mentions the classic painters but hardly mentions any of his contemporaries.
When he died in 1890 he was just on the verge of being discovered, so I must agree with you when he put him in the category of Post impressionism.
You hit it on the head when you writedidn't have a close common approach or agenda.
For Van Gogh is his own category but its still a branch of Impressionism.
Its correct to put Van Gogh in the same category as Seurat. Van Gogh experimented with Seurat's patterns and admired him for his mixture of color.
But Van Gogh was a contemporary of the impressionist painters although he really didn't daddle with pastels because he felt he didn't have the knowledge to do it. But Van Gogh loved combining colors that didn't match in other painters conceptions and loved the colors gold and blue, which he used to portray the sky.
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What is amazing is that so many of his letters survive today and well catalogued. When you read them, you can read of how he viewed colors and which ones he thought to be the ones he could best transfer to a canvas and ones/combinations of colors he thought difficult.
This has nothing to do with the rest of this post or thread but interesting.
http://www.smh.com.au/entertainment/art-and-design/scientists-solve-my stery-of-van-goghs-colours-changing-hue-20110215-1av6w.html
I love this quote by Gauguin
A word of advice," he wrote to Schuffenecker, "don't copy nature too much. Art is an abstraction; derive this abstraction from nature while dreaming before it, and think more of the creation which will result [than of the model]. This is the only way of mounting toward God—doing as our Divine Master does, create."
He wasn't interested in trying to paint what he saw but more interested in translating nature the way he saw it into a piece of art. This was what Van Gogh obvious did too. They considered reality as abstractions, ever changing and ever being translated, also by the same person dependent upon their moods and present desires.
What Van Gogh did, consciously or not, was that his moods determined the painting he was doing. He mental state influenced his paintings and sometimes his choice of colors mirrored his depressions.