Python 3

def minion_game(s):
    V = frozenset("AEIOU")
    n = len(s)
    ksc = sum(q for c, q in zip(s, range(n, 0, -1)) if c in V)
    ssc = n * (n + 1) // 2 - ksc
    if ksc > ssc:
        print("Kevin {:d}".format(ksc))
    elif ssc > ksc:
        print("Stuart {:d}".format(ssc))
    else:
        print("Draw")

Several optimizations.

1. Use frozenset for the vowels. This is the fastest to lookup in.
2. Iterate through characters instead of indexing
3. Iterate q reverse to avoid a subtraction in each sum
4. Avoid repeated calls to len
5. Only calculate Kevins score the hard way. Use the fact that the total number of substrings is n * (n + 1) / 2 and find Stuart's score by subtracting Kevin's from the total number of substrings.