It's always hard to prove a negative, but this appears to be in the realm of fantasy and does not correspond to the modus operandi of a PAVN ambush. Local variations did occur and improvised ambushes could take place if an opportunity was found, but an ambush along the lines that you propose is unfortunately highly ineffective. The practicality of having all soldiers rise at the same time, weapons at the ready, without being noticed seems highly dubious. Add to that the fact that their vision would be impaired due to all the dirt and sand in their eyes and you have a recipe for disaster.

If we can discount this method at a glance, what would a PAVN soldier have known as an 'underground ambush'? To answer this, we're going to turn to a primary source from the first year of the American escalation in the Vietnam War. The following information is drawn from an interrogation with a North Vietnamese soldier from the 334th battalion, 32nd PAVN regiment who infiltrated to South Vietnam in January 1965. This soldier, 23 years old at the time, was captured in November 1965 by the ARVN.

As part of his advanced training as a PAVN soldier, the soldier in question was trained in two specific ambushes: Hit-and-run ambushes and so-called 'underground' ambushes. In theory, these two ambushes were meant to be used at two widely different times; the first being used to ambush a convoy or groups of vehicles while the second one being used to ambush patrols on foot. However, adaptability was stressed according to the account of the soldier, stating that they were instructed in performing a hit-and-run ambush by lying down on the ground, covering themselves with leaves, and wait for a pre-determined signal (more on that in a moment) for them to rush out. In both cases, the soldiers were never meant to be positioned on the same paths as American soldiers. They were always meant to be flanking them for the best effect.

In contrast to the hit-and-run ambush, an underground ambush had its starting point (you guessed it) underground. On each side of the ambush zone, each soldier would dig their own individual underground trench of around 1 meter of depth. Additionally, the account tells us, the "trench exit was rectangular, 0.80mm x 0.60mm in size, and was camouflaged with a bamboo-wattle which was covered with leaves or grass depending on the earth color." The earth from the trenches would be strategically spread in the immediate area or in a nearby river/lake/pond if there was one. Once the ambush was set, it would be triggered by a pre-determined signal. In this example, the commander of the unit, at a distance, would shoot 3 times with his pistol.

Both the underground ambush and the hit-and-run ambush could be combined and often were to cover all exits and escape areas. There were often additional units covering routes that might be used by relief troops. If, for example, three companies trigger a conventional roadblock attack, two platoons could be used to intercept and ambush relief troops as well as support the main attack. No matter what, camouflage would be of the most paramount importance and would be widely applied to hide any and all indications of an impending attack.

In sum, the PAVN soldier was trained in a variety of ambushes. The purpose of these ambushes were meant to be deceptive and hidden but were all tightly coordinated. They would never be placed in the immediate path of the intended target, but soldiers could be prepared to strike while being in underground trenches or so-called 'spider holes'. Additionally, we need to keep in mind that this source was captured in 1965. PAVN tactics did change over time and local conditions and experiences played an important role in shaping how ambushes were carried out or what training soldiers received in performing them. Furthermore, ambushes could be countered and soldiers in underground trenches were at risk to be found in a vulnerable position by the same soldiers they were trying to ambush.

Source:

Interrogation of a member of the 334th battalion, 32nd regiment of the North Vietnamese army, 23130002025, Folder 002, Box 30, Douglas Pike Collection: Unit 05 - National Liberation Front, The Vietnam Center and Archive, Texas Tech University.