The reason as to why Kim and Trump decided to hold a summit in Vietnam is because It is an ideal location for many reasons.
First Vietnam has diplomatic relations with both the US and North Korea, despite once having been enemies with the US – and could be used by the US as an example of two countries working together and setting aside their past grievances.
Ideologically, both Vietnam and North Korea are communist countries, though Vietnam has over the last few decades opened up to foreign investment and emerged as one of the fastest growing economies in Asia.