Sorry to disappoint you - because modern English is a "mongrel language", it takes spelling rules from both Germanic and Latin (via French) origins. Then there's Scandinavian and Gaelic influences... It will be a bit easier once you learn more words - you'll be able to group them based on where they come from, and you'll start to see the consistencies in spelling.

All English words must contain at least one vowel. That's the only rule that is applicable to all words, I think.

There are quite a few rules. None of them apply to all words.