This study investigated Korean college students' performance as measured by two different vocabulary assessment tools (the Productive Vocabulary Levels Test (PVLT) and the Productive Vocabulary Use Task (PVUT)) and the relationship these assessments have with students' writing proficiency. A total of 72 students participated in the study. The students were asked to take the PVLT and the PVUT and write an essay. They were also asked to write a reflection paper to assist in examining what kinds of processes they go through in vocabulary production for both tests. The results of the study indicated that the students' ability to produce vocabulary and the number of lexical errors displayed in the results of the two different assessment tools were highly contingent on the test format. The students produced more target words in the PVLT since they were helped by clues of a few given initial letters of the target items. On the other hand, the students produced more alternatives than target words when allowed to choose words freely in the PVUT. As for the relationship between the students' performance on the assessment tests and their writing proficiency, the results of the study showed that the students with a higher proficiency produced a higher number of correct target items and left fewer words unanswered in both tests. This study explores the possibility of using an alternative assessment tool and suggests that careful interpretation of the results of the different tests should be associated with assessment purposes.