clintonforever,
As Aranfan pointed out, Tet both destroyed the Viet Cong (VC) a military force and won the war for the Viet Cong in the US media. It was a perfect example of what wars actually are: politics by other means. North Vietnam won the war politically which is the only way you truly win wars.
(As an aside and to forestall the usual bleating these threads engender, IMHO, the US had no business being involved in what was essentially a Vietnamese civil war.)
After Tet, the US and ARVN weren't fighting the VC no matter how much the propaganda on both sides wanted you to believe. With the VC smashed and recruiting in the south greatly hampered, the military struggle depended almost wholly on North Vietnam Army (NVA) regulars infiltrated into South Vietnam.
With the DMZ sealing off the official border between the two sides, infiltration of and supplies for NVA units needed to pass through Laos and Cambodia along a LOC you may have heard of; the Ho Chi Minh Trail. The use of this LOC by North Vietnam and the the US' and South Vietnam's understandable desire to cut it quickly led to an expansion of the war into Laos and Cambodia, first by special ops and air power, then with ground troops, with horrific consequences for both those nations.
If the VC don't launch Tet, they're still being slowly militarily defeated by the US and South Vietnam and will eventually attempt a politically-focused solution; i.e. another Tet or Tet-like offensive. Also, once VC strength and effectiveness falls below a certain point, North Vietnam will begin to infiltrate regular troops to pick up the slack. Putting it simply, Tet merely sped up a process that was already happening and would continue to happen.
Bill