While Vegeta has a reputation for being a tough guy within the Dragon Ball universe when it comes to training the younger generation of fighters, his tutelage is nothing compared to that of Piccolo. In fact, the hardships that Gohan had to endure while training under Piccolo proves it.
Even though many fans are more familiar with him being one of the most heroic members of the Z Fighters, Piccolo was originally introduced in the Dragon Ball universe as a villain who went by King Piccolo or the Demon King as he believed himself to be a literal demonic entity as opposed to what he actually was: a Namekian. In fact, Piccolo didn’t begin his journey from villain to hero until Goku’s long-lost brother, Raditz, came to Earth and started causing trouble for Goku and his family. In an effort to kill Raditz and cement his position as one of the most powerful beings on Earth, Piccolo teamed up with Goku, and together they were successful in defeating the Saiyan villain. Unfortunately, Goku was also killed in the process–a situation that was made worse by the fact that Raditz’s Saiyan partners were on their way to Earth as well and would arrive in a year. To prepare for this attack without Goku, Piccolo decided to train Goku’s son, Gohan, and he did so in an immensely grueling fashion.
In Dragon Ball chapter 207 by Akira Toriyama, Gohan officially begins his training under Piccolo, but before the Namekian teaches Gohan any fighting techniques, he first decides to leave the half-Saiyan child alone in the wilderness for six months. After those six months are up, then Piccolo will teach Gohan some battle moves and ki blasts during the remaining six months before the Saiyans arrive at Earth. While it is initially hard for Gohan to adapt to this sudden change in his life given the fact that he had never been exposed to any form of training, martial arts or otherwise, he does get tougher as the months go by. This training sets him on the course for the rest of his life as one of the strongest fighters Earth has ever seen.
One thing to note about the Piccolo/Gohan training situation is: Gohan is four years old. Piccolo left a four-year-old to fend for himself in the wilderness where there are literal dinosaurs roaming around trying to eat him. Not only is Gohan four, but he has no fighting experience with which he can use to defend himself from deadly threats, mostly due to the fact that he’s four. Then, when those six months are up, Piccolo doesn’t hold back in the slightest when sparring with the untrained Gohan as he just beats the living crap out of this four-year-old kid he had just left stranded alone for half a year. This fact alone makes Piccolo more hardcore than Vegeta no matter how cruel Vegeta’s methods might seem on the surface.
The most notable training sessions Vegeta has led was with his son, Trunks, in the hyperbolic time chamber and with a Saiyan named Cabba during the tournament between Universe 6 and 7 in Dragon Ball Super. In both cases, Vegeta is mean, unrelenting, and overall ferocious in his methods in an effort to push his students to the brink of death and enrage them to the point of murderous fury to unlock the inner power of their Saiyan heritage. However, all of them were older than four, and Vegeta didn’t begin his training sessions by putting his students through a six-month stretch of total isolation–proving that Piccolo’s Dragon Ball training is unquestionably tougher than Vegeta’s.