Trump walking away was the right move. Going to Vietnam with no deal in sight was fucking stupid.
It's been clear from the outset that Kim was after sanctions being lifted in exchange for promising to take the first steps towards beginning a process that might eventually lead to something vaguely similar to denuclearization without any actual impact on DPRK's nuclear capabilities. Because that's how the Kim family rolls. That Trump was willing to talk at all suggested that he was willing to drop sanctions in exchange for promises (sure to be broken) that he could trumpet as a win, so I'm a bit confused as to why he'd have gone to the summit in the first place if he was going to walk out on that sort of offer. I know, the man is convinced of his own god-like negotiating capabilities and delusionally thought threatening to walk out would get a better offer from Kim.
So far as I can tell threatening to walk is the sum total of what Trump knows about negotiating strategy.
"I am gonna walk out if you don't give me what I want" is a small-minded person's tactic. If you are negotiating small things, it's a fine tactic. It works if you are negotiating with a guy over scalped Yankees tickets, or you're buying a Honda Civic. If you're negotiating something of enormous importance, complexity and nuance, where your adversary has done thousands of hours of background research on you and knows what your tendencies are, you'd better have a lot more tools in your box.