I discussed lobotomies and their history in a previous answer here, which though focused on when people realised that the frontal lobotomy was problematic, does in a roundabout way also answer your question, in terms of what they believed about the brain at the time and why they thought it might be a good idea. Often it was done to people with quite severe mental illnesses who were institutionalised, and it often did treat the mental illness in a certain way, because they often did stop exhibiting the symptoms of their previous disorder. It usually made them easier for the staff to deal with in the settings where mentally ill patients were institutionalised; these can certainly be chaotic places. But ultimately the cure was worse than the disease in most cases.

I wouldn't necessarily call most people who'd had frontal lobotomies 'vegetables' - more typically, they were generally still aware and responsive, but they were generally very passive, apathetic, unable or unwilling to make plans. These side-effects may be why the pre-frontal lobotomy could be successful at times, because there is a certain 'can't turn off the bad thoughts' factor to some mental illness, and the lobotomy clearly (at great cost) shut the bad thoughts down. But some people, especially where the lobotomy was carried out incorrectly and visceral parts of the brain were accidentally removed, had more serious deficits as a result of the lobotomy that might be more characteristic of what you mean by 'vegetable'.