With the benefit of hindsight, it seems clear that there would have been no real cons to withdrawing from Vietnam after the Tet Offensive. However, US leaders would have had a hard time realizing that at the time.
At the time, withdrawal would have seemed like an admission of weakness. US leaders would have felt that they were losing credibility with their allies around the world. These allies would have felt that the US might abandon them if it would abandon Vietnam. This would have seemed like a compelling reason to stay at the time. In addition, there was still hope and confidence that the US could win the war. Withdrawal would have precluded this.
In hindsight, we know that the US would withdraw from the Vietnam War 5 years later. This did not lead to a loss of global confidence in the US. Withdrawal in 1968 would probably have not caused such a result either. If the US had simply withdrawn at that point, large numbers of human lives and huge sums of money would have been saved.