So to summarise:
Being a vegetarian in China is easy and exciting and interesting if you are self-catering. For those dependent on restaurants, street food etc it is difficult, but not impossible.
The problem is partly a language barrier issue - few restaurants have English menus or English speaking staff - but even Chinese speakers (native or otherwise) struggle.
Traditional Chinese cuisine and home cooking is still largely vegetable based, but not vegetarian. Animal fats are widely used for frying and animal products often added to pimp up the veg.
You won't starve in China, but you have to be prepared to work at finding acceptable food. Some people take that on; others give up and leave.
Try to see the cooking being done. Sometimes possible - not always. Point out what you want - not what you don't want. But be wary of animal fats being used and what is that strange looking 'spice' being added? Pork floss? Shredded dried shrimp?
I have deliberately avoided recommending specific dishes. What is available varies enormously from place to place. The few dishes or foods I have specified are those I think are available pretty much everywhere.
I have no experience of it, but I think being vegan in China would be very problematic unless you were self catering. It's not that the food isn't there. It most certainly is. Veganism is just not understood in China at all.
Finally, let me relate an anecdote.
A few years ago, I was in a little-known town near the Guangxi - Yunnan border. I was invited to a full scale banquet. (My heart usually sinks when that happens - the food is usually second rate at best. People are only there to get as drunk as possible as quickly as possible.) But to decline the invitation would have been inexcusably rude, so along I went.
The food was divine. The hosts and guests were polite, sober and interesting. It was, without doubt, the best banquet I have had in near-on 20 years in China. And it was entirely vegetarian.
Or so they told me!