The reason why modern boxers don’t use a vertical fist to punch? Because the benefits of punching with a vertical fist are neutralized when wearing gloves. The hand is already protected. Blows can be thrown with more power because they have the increased energy of momentum behind them, as well as the weight of the gloves themselves. Gloves can weigh anywhere from 8 to 20 ounces, which is significant when adding power to a punch.
Additionally, because boxers needn’t worry about breaking their fists, they can afford to throw with increased power.
Second, you said. “I have never seen or heard of any professional boxer or fighter who uses any soft martial art techniques in the ring.” Both Judo and Jujitsu are “soft” style martial arts and both are used in MMA. I personally don't think any martial arts technique is "soft", but that is a matter of definition.