d = {}
for word in words:
    if word[0].lower() in 'aeiou':
        d.setdefault(word[0].lower(), []).append(word)
index = {}
r = {index.setdefault(word[0].lower(), []).append(word) for word in words if
    word[0].lower() in 'aeiou'}
print(r)
print([(k, len(d[k])) for k in sorted(d.keys())])
print([(k, len(index[k])) for k in sorted(index.keys())])
Append to a dict of lists with a dict comprehension