The new year will bring the first threat of fines under Denver’s new vacation rental rules, and there appear to be plenty of potential targets. Six months into the city’s roll-out of short-term rental licensing, most people who rent rooms or homes on online services such as Airbnb and VRBO still have not gotten on board. That’s not a surprise. San Francisco, Portland, Ore., and other cities similarly have struggled to get most hosts to start collecting lodging taxes and obtain a license or permit. Such lags often persist for years.But next week, Denver licensing officials will begin holding out the threat of fines — topping out at $999 for repeated violations — though they say violations still will be met first with a warning.