I'm not really concerned with food standards because that's an issue which can affect any food outlet no matter what they do. It's a question of socioeconomics, which is probably best not gone into here in detail. I try to avoid using any high st shop, I try as much as possible to use independents, luckily I live in a large city with plenty of those. I can't singlehandedly destroy capitalism but I can choose what to support and whether they sell meat only comes into it when I decide whether or not to buy something from a place that mainly sells meat.
Say Burger King. They sell several veggie items (don't know about vegan) and I have eaten there in the past. But I don't, because it's Burger King and I heartily disapprove of that kind of business, model, structure and the society that makes it exist as it does. Those companies trying to attract new customers by ''supporting veganuary'' are of that sort. I don't use them, ever, anti-capitalist killjoy that I am, and IMO nobody should. But the fact they sell meat is to me less of an issue than the fact they are (kinda) a symptom of Everything Bad In The World.
More vegan food around? Good. But there are bigger issues around those companies and giving them promotion and applause for doing what they should have been doing anyway is a big distraction from the fact they should all be boycotted full stop.