According to the BHF vegetable oils (the trans fat free ones) including those in avocados are good for the heart.
If you are extremely careful to ensure that you get all the vital nutrients and sufficient energy you should be OK on a vegan diet, but it's easier to achieve that on a vegetarian one.
Typical western diets, especially those which include processed and industrially prepared foods, in particular products from intensively farmed animals, are certainly not healthy. However according to the material I've read (in connection with controlling inherited diabetes and heart issues) vegan and vegetarian diets which contain industrially processed and prepared food, and some plant products grown using intensive farming techniques can be as unhealthy as any animal derived foods.
What saddens me is that elsewhere in the world, as people get wealthier, they aspire to leave behind their healthy foods and start eating western style rubbish instead, with the inevitable deterioration in their long-term health.