There is a long list of things that Vans (more specifically VF) will use to fire an employee. Saying “Fuck you” to a customer is so obviously an infraction of their service standard that I don’t even think it’s actualy outlined on that list. It’s considered common sense.
Side note: Taking in to account the company’s Orange County roots (historically Southern California’s most conservative county), it shouldn’t be a shock to most of you that the majority of the highly paid executives behind the scenes at Vans are Trump supporters. A lot of people still choose their party based on a short list of political issues, namely tax rates. This guy would have been fired regardless of what hat the kid was wearing, but anyone expecting Vans to take a moral stance against Trump supporters will be ultimately disappointed.