When we enter name in Name/lookup ID field, name is broken into firstname and lastname before performing search action. But in case of “Van Nguyen”, it will not get broken because  “VAN” can be the beginning of a last name and so “Van Nguyen” is considered to be a last name. There are many other strings which wherever found in the name will be considered as the beginning of a last name. Below is a list of other names considered to be part of last names. 

VON, VAN, DE, DU, DA, DI, LA, LOS, ST, ST. DER, LE, MC

Alternate solution: Enter Van in the first name search bar and Nguyen in the last name search bar to find the constituent.