Do you want a regular fitted cardi style that's sleeveless and has a button closure? You realize you might find something suitable with sleeves which you may adapt. How long is "long"? - hip - knee? Patterns are rated from beginner - experienced. How much can you handle? I've looked through some retro patterns from way back but didn't find anything suitable. I have a Laines du Pingouin book where everyone looks like Audrey Hepburn. It's titled "the new Paris fashions" with 21 patterns of sweaters, suits, coats etc. It cost 75 cents so how old do you think that is? Here's a link to a recent one that fits some of your description. Lion Brand® Nature's Choice, Organic® Cotton Pattern #: 80453AD pleated tunic. You can forget the sleeves and probably just do a single crochet around the arm holes to finish them off. It is a pullover so it might not fit the bill. I'm not sure if you want a button up cardi.This is done in a cotton so it's probably for spring/summer wear. . . or, perhaps you'd like something like this one that I'm going to do. It's popular these days and it certainly could be retro too. Everything that was old is now new and vice versa.
Check out garnstudio.com pat. #119 plus some of their others. They have several tunic styles you might like. If not give a more specific idea of what you want to do.
Good luck. Jean