To me, Vance's pronouncement seemed to say that Afghanistan might be winnable by someone else at some time in the distant future. These days generals don't seem to wage wars they might have to win or lose before they can safely retire. But they always know that somebody can win it, eventually, if they only have lots of time, and focus and patience.
Of course a lot depends on how you define 'win'. When we got into this adventure almost a decade ago, 'win' meant we would 1) drive the Taliban out, 2) track down bin Laden and destroy al Qaeda, 3) establish a viable, democratic central government, 4) raise, equip and train an Afghan National Army, and 5) institute Western liberal human rights and, in particular, women's rights.
Nine plus years later, how have we fared? The Taliban are resurgent; bin Laden is apparently alive and keeping busy in Pakistan; al Qaeda has spread throughout the Middle East and North Africa; we have a criminal enterprise in lieu of a viable government in Kabul operated by a totally corrupt bureaucracy, judiciary and security service; the country has morphed into a narco-state; Afghan soldiers desert at roughly the same rate as we manage to train them; and the warlords we empower have restored Islamist fundamentalism to the countryside crushing all the nonsense about women's and human rights.