I believe you are incorrect. Flusser describes side vents as an English traditional look. Side vents were originally designed for wear when riding horses, so there is nothing modern about it.
Very seriously, you have 100% no idea what you are talking about.
Okay fair enough, here's my explanation. American suit styles are tradtionally a single vent down the middle. English are common for double. For an American suit to have a double vent its more of a modern look. Now that doesn't mean it's fashion forward what so ever. I just mean that traditionally single or no vents with American suits.