The Vietnamese had room to maneuver and hide and had the support of the locals. In addition you overestimate how little the Chinese government gives a shit about the average citizen. In such an engagement there will be no "hearts and minds" war. If they decide to go the military route they will flood HK and massacre anybody who gives off the slightest whiff of sedition.

Personally I think they'll just continue arresting and making people disappear to the mainland so there is nothing damaging on the news.

I'm glad that you asked. We had a long history with our big "brother" China. But I don't think this is relevant because at the end of the day, Hong Kong and Taiwan are still Chinese.

For a more recent example ISIS was crushed.

The Vietnamese had massive foreign support. China is unlikely to give up so easily, and no country is going to want to keep arming such a resistance.

In all fairness the Vietnamese had among the most battle-trained armies of the time, with massive manpower, enough of weaponry and extreme determination. Plus a difficult terrain. Equipment-wise they had infantry weapons, artillery, air force, aerial defense...

HK has maybe a difficult terrain?