To go into this even further, think about the possible solutions for each:
gr_p_ - You can safely assume 'e' is at the end given the structure of the word; from there you only have to pick between 'grape', 'grope', and 'gripe'.
__a_e - Could be tons of words! 'Grape', 'crane', 'frame', 'trade', 'grade', 'brake', 'craze', 'chafe', 'phase', 'shale', 'whale', 'drape', 'shade', 'shame', 'shape', probably even more I'm not thinking of.
You don't want to focus exclusively on either vowels or consonants, and when considering the balance you have to take into consideration that vowels will provide much more in the way of context clues.