But there are different sets of rules that could govern the information that you glean from the four cards. The simplest rule could be that all the cards have been given random assignments. The four cards would still represent the rest of the deck. All kinds of rule sets could be represented in any four cards as long as you only get to see four cards.

We are have one given, "these four cards represent the rest of the deck" but at no time are we told that if "If a vowel is printed on one side of the card, then an even number is printed on the other side" than the deck must all be arranged such that ALL cards with a vowel must have an even number. There are other assignments that could hold true. I'm sure they meant it to mean that you can confirm that all cards with a vowel must have an even number which can be confirmed by turning up two cards but if you are putting up a logic puzzle than you should state you givens in a super hard way.

The second I see this kind of thing I get a the same frightened feeling that I got taking structured logic exams. Leave out a given that seems obvious and you fail the problem. If you turned up an even without a vowel you could toss this shit out but if you turned up even's with vowels you still can't confirm the rule.