It would certainly have been very difficult to have won the Vietnam War. Moreover, it would have been very difficult to even know that we had won the war. This is because the war did not have an obvious ending point that would have made it clear that victory had been achieved.
In a conventional war like World War II, it is easy to know when one side has won. The winning side takes so much territory that the losing side realizes it has lost and surrenders. There is no doubt that the war is over.
In the Vietnam War, however, this would not have happened. Victory would have consisted of making the Vietcong insurgency (and its North Vietnamese backers) give up the fight. The problem is that it would have been very hard to know when the fight had actually been abandoned. Progress might have been measured in terms of declining numbers of attacks by the Vietcong. It could have been measured by the increased popularity of the South Vietnamese government. However, it would have been hard to know if decreased attacks actually meant that the Vietcong were losing or if it just meant they were lying low until a better time. It would have been very hard to measure the degree of public support that the government enjoyed.
Because Vietnam was not a war of territorial conquest, it would have been very hard to know if we were actually winning the war.