This study aims to compare the effects of four types of FFI on second language vocabulary learning. To do so, the study adopted a quasi-experimental pretest-posttest design, including five groups, each receiving a distinct treatment. The participants were 80 fourth-grade male students ranging in age from 17 to 19. Before the treatment phase, the participants took a researcher-made test of vocabulary as a pretest which was meant to measure the participants’ prior knowledge about the target words. After the treatment phase, the participants took a researcher-made test of vocabulary as posttest to measure the students' achievement of the target words. Independent samples t-test and one-way ANOVA were run to analyze the scores from the pretest and the posttest. The results indicated that there was no statistically significant difference between proactive and reactive FonF in the comprehension mode in terms of promoting vocabulary learning. There was also no statistically significant difference between these two forms of FonF in the production mode in terms of promoting vocabulary learning. However, there was a statistically significant difference between the comprehension-based and production-based FonF in enhancing vocabulary learning. These results extend the positive effect of FFI to L2 vocabulary teaching/learning.