That was excellent. You effortlessly caught the way things are between Nicholas and Vash, and how quickly they fluctuate. I really liked the last line, and Nicholas' instinctive, emotional understanding that he won't ever figure Vash out, and he just has to take Vash on faith.
One note on punctuation g - names (and nicknames) need to be set off by commas when directly addressing someone in dialogue. For example: "Thanks Tongari" should be punctuated: "Thanks, Tongari." You've done it correctly in some parts of the story, but not consistently. Just a heads up. :)