Honestly... giving people breaks. Obviously being completely vegan "no exceptions" is more impactful than occasionally slipping up, or by being vegetarian, or by being a "Tuesday vegan" or whatever. But people get very, very defensive about their dietary preferences, and a person doing something is better than doing nothing, so you gotta give people breaks sometimes. When you hop into conversations preaching veganism with (I'll be frank) an air of superiority, you will make some contrarians reading the conversation shift their mindsets from "maybe it's okay to eat vegan once or twice a week" to "fuck vegans, I'm eating a steak at every meal".

So you gotta be careful. You need to praise people for doing what they can, and you can't shame them for "not being vegan enough". Because the purity-testing BS will turn people against you.

(This is a level of tact that extends beyond online discussions on diet, and if it's a skill you haven't learned yet then it might serve you well to practice it. Catch more bees with honey than with vinegar, etc.)

As for proactive things you can do... honestly, learn to cook delicious vegan food that appeals to omnivores, and then serve it to them. Bonus points if they don't know it's vegan until you tell them. There's this thing in teaching called a discrepant event, which is when you shock someone out of their preconceptions by presenting them suddenly with a surprising situation which challenges their assumptions. In this case, the assumption would be "vegan food is boring and gross" or "a meal doesn't feel complete without animal products in it". So don't give them a salad. Learn to make an awesome, delicious curry with some vegan naan, something hearty and filling... serve it to them, let them give you accolades about how great your cooking is... and then tell them casually that it's vegan. Surprise them into learning that eating vegan doesn't have to be a sacrifice.