Both Ho Chi Minh and Vo Nguyen Giap seem to have had serious reservations about the plans for the 1968 Tet offensive:
"Giap has often been assumed to have been the planner of the Tet Offensive of 1968, but this appears not to have been the case. The best evidence indicates that he disliked the plan, and when it became apparent that Le Duan and Van Tien Dung were going to push it through despite his doubts, he left Vietnam for medical treatment in Hungary, and did not return until after the offensive had begun." [44] Pribbenow, Merle (2008). "General Vo Nguyen Giap and the Mysterious Evolution of the Plan for the 1968 Tet Offensive". Journal of Vietnamese Studies 3 (2): 1-33...
http://en.wikipedia.org/wiki/Vo_Nguyen_Giap
Unfortunately, only the first page of Pribbenow's article is available online for non-subscribers.
http://www.jstor.org/stable/10.1525/vs.2008.3.2.1?seq=1#page_scan_tab_contents The fullest summary of it I have been able to find comes from a Trotskyist website:
"Based on 'a number of Vietnamese histories, documents, and sources have become available' in recent years, author Merle Pribbenow shows that as Tet 1968 approached, Giap and many leaders, including Ho Chi Minh, had great reservations about the idea of the offensive, as they believed the conditions were not ready for popular uprisings in the cities (Pribbenow, Journal of Vietnamese Studies, 2008, vol. 3, issue 2, pp. 1-33, for this and the paragraphs below regarding Tet).
"As background, in June 1966, the Politburo war subcommittee met with the Central Military Party Committee and outlined a plan to win a decisive victory. It consisted of 'main force' units attacking the enemy on four main battlefields, to be combined with attacks and insurrections in three main cities.
"While Giap agreed with fighting 'big battles' as one part of an overall strategy, he stressed that 'the attacks on the cities should start with small military attacks and only gradually build up to the 'insurrection' stage in certain specific areas and cities, once communist military forces had attained local superiority in those areas'.
"In January 1967, the Central Committee approved Resolution 13, which called for 'an all-out effort ... to win a decisive victory in a relatively short period of time', by inflicting heavy casualties on US forces, destroying a major portion of the South Vietnamese army, before inciting a 'general offensive--general insurrection' in the cities--there was no consideration given to launching urban insurrections before some decisive military defeats of the enemy, understanding this would mean the revolutionary forces would be smashed by the US military.
"The resolution made clear that this should occur 'at end of a gradual, step-by-step process: 'In coordination with the political struggle, [we must] build up our strength, gradually and systematically gain control in the cities, weaken the enemy's controls, intensify our movement in the cities, and create conditions to enable us to advance toward launching a general offensive--general insurrection.'
"Resolution 13 underlined that the aim of winning such decisive military victories was to force the US into negotiations--which they considered urgent given the upcoming US elections in 1968--in a situation where the Vietnamese would thus be in a stronger negotiating position. The party thus had a political-military-diplomatic strategy.
"However, there was a basic problem in the plan--their military forces were not yet in a position to impose such massive defeats on the American military, in particular quickly enough to meet the timetable for an urban offensive/insurrection at Tet--Vietnamese Lunar New Year--which was to fall in late January 1968. General Van Tien Dung, recognising this problem, went behind Giap and conferred with Le Duan, who proposed the entire set of intermediate stages, including military victories, be dropped, and instead to move immediately to the final stage of general offensive/insurrection. The insurrection would coincide with main force military attacks all round the country to divert the enemy, but would not need to wait for them to cripple the enemy's forces.
"Le Duan was thus proposing a radically ultraleft 'insurrectionist' strategy, one that had no relation to either Moscow or Beijing views, and no relation even to the party disputes, since it called for ignoring not only popular support and guerilla struggle, but even main force military victories--but with the aim of forcing the Americans into negotiations.
"When the plan was put to the Politburo, there was significant opposition, including from Ho Chi Minh, who stressed, among other things, that they still needed to pay attention to fighting a protracted guerilla war, and that 'we must also pay attention to the need to preserve the strength of our people. If our people and our resources become exhausted, then we will not be able to fight, no matter how many troops we have.' Due to reservations of many members, Politburo approval of the full plan was delayed until December 1967, when both Ho and Giap were out of the country for medical treatment--Ho in China and Giap in Hungary.
"Indeed, Giap was allegedly so opposed to the plan that he extended his stay there until just after the offensive begun, to underline his displeasure with this course. Once in operation, he returned to lead it as well as he could. General Tran Van Tra, who led the southern National Liberation Front forces, those who were decimated in the offensive, later criticised the northern leadership for not giving the NLF sufficient time to prepare for the offensive.
"While it is true that the political impact on the US government and anti-war movement was ultimately enormous, it is also true that Le Duan's forcing of the 'insurrection' did not lead to urban uprisings (though it was more successful in many smaller rural towns), and did lead to a US counter-attack which decimated the southern revolutionary forces. In Hue, both the excesses of the incoming revolutionary forces [1], and the extremely vicious US air attack, imposed no less than catastrophe on that city.
"It is a very difficult thing to look back and suggest how things may have turned out better if the Ho-Giap strategy had been adopted instead, and the offensive was thus delayed. But while Giap, as a loyal party member, always publicly defended the offensive for its impact politically, the facts show he cannot be blamed for a strategy that resulted in so much death."
http://www.internationalviewpoint.org/spip.php?article3168
I can't be certain how accurate this is as a summary, or for that matter how accurate Pribbenow's original article was, but he is a retired CIA expert on Vietnam who appears to be a respected scholar; see for example his writings for the Wilson Center at
http://www.wilsoncenter.org/staff/merle-pribbenow
So suppose both Ho Chi Minh and Vo Nguyen Giap are in better health in December 1967, instead of being out of the country for medical treatment, and insist on scrapping the Tet Offensive as we know it for a more gradualist strategy? (To begin with--does LBJ decide to seek another term?...)
[1] A polite way of referring to the events described at
https://en.wikipedia.org/wiki/Massacre_at_Huế