The accentedness of nonnative speech has previously been researched as an effect of L1 transfer. However, no previous study has investigated whether the comprehensibility of nonnative speech, especially that of vowels, changes depending on the gradation of an accent. To this end, this study compared three levels of accented speech to examine how accentedness and vowel similarity affect native listeners, as measured by the real word advantage in a lexical decision task. Fifty-six American English listeners were exposed to three levels of Spanish-accented English: a control native speaker, a low-accented L2 speaker, and a high-accented L2 speaker. The participants listened to 120 real words and nonwords and answered whether they were English words. These items included vowels that are similar in English and Spanish, and vowels that exist in English, but not in Spanish. The results showed that, as accentedness increased, word recognition became harder. The vowel similarities in English and Spanish were not found to be significant. The results of the study indicate that accentedness is not simply a consequence of L1 transfer; rather, it can be variable, and this variability can affect how well foreign-accented speech is understood.