I think improving your diet, makes you healthier. (as well as a million other things!)
Being vegan doesn't mean your diet will become better, you could live off of chips and crisps and be vegan but very unhealthy. You can be vegan and malnourished, or you can be extreemly healthy and vegan.
Lots of vegans (through choice) have a big intrest in healthy eating - or that's what I've obserevd over the internet, and by the fact most vegan products are advistised heavily by their health benifits. However, there is of course an intrinsic value in not eating animal products due to eating less saturated fat and bad cholesterol(sp?).
There's also health benfits having much less unhealthy choices, and being "encouraged" (due to lack of options!) to cook a lot of food from scratch (which results, usually, in a lower salt, sugar, saturated fat and fat intake in meals).
However, does veganism improve health? No, not nessersairly. Does eating vegan correlate with eating healthier, I don't know statistically, but I'd guess so.
Also, I always think with questions like this, many aspects of veganism have absolutely nothing to do with health: using vegan shampoo, not wearing wool and leather, not buying leather sofas, not buying lipbalm with beeswax in it. So even if a vegan diet is healthier, there's much more to being vegan than just diet.