Vindictive means vengeful. This means you don't let wrongs go unpunished. This is turn means the wrongdoer isn't allowed to go on committing wrongs with impunity. Someone who fails to be vindictive is partially responsible for any further victims.
However, peasants are not, in general, vindictive. If they appear vindictive, they are in fact spiteful. The wrong is merely an excuse to cause harm they wanted to cause anyway. Many non-wrongs also form such excuses. Since the other option is generalized spite and resentment, a non-"vindictive" peasant is simply not unpleasant.
The instinct for revenge generally isn't a revenge instinct, but instead a instinct to spite.
In practice, revenge is something that must be done rationally, and carrying it out expends willpower. If you need to take revenge, it means someone is capable of harming you. Much easier to avoid them rather than risk another confrontation. Even if your revenge succeeds - and even deters further deviance - it may form an excuse for spiteful allies of the vengeance-inspiring defector. Build a fence and then "live and let live," right guys?
If revenge seems to come naturally, all you're doing is enabling the spiteful. In the worst case, you may even be prodding the peaceful into acting spiteful so as to appear to fit in.
I shouldn't have to say the following, but probably do. Punishment isn't vengeance. If you can inflict pain with impunity, it means you have military superiority. If you have military superiority, you don't have to let them damage you in the first place, and [revenge] is a meaningless concept.
If you first have to achieve military superiority, as by conquest, before you can take revenge, then you can't take revenge, because the pre-requisites make revenge meaningless. "Hey, if you do that again, I'll have you executed." The point is deterrence, not to cause pain. If they do it anyway, execute them to encourage les autres. The issue is done with and you can put it out of your mind.
By contrast, a peasant who gains military superiority will usually turn to spite. Some mistakenly believe post-facto punishment is necessary. The peasant, however, won't allow one punishment to clear the slate. They will find some excuse to continually harass anyone under their power. The point of spite is pain, not deterrence.
Peasants generally don't go in for killing. Mobs do sometimes, and crazy peasants, but killing is too responsible, so peasants shy away from it. Even a peasant knows that, "I didn't mean to!" isn't an excuse when someone is dead. E.g. in mobs you can always say it was the other guy, who in turn can blame a third... Peasant instincts don't really get security cameras, unfortunately.
For low-level spite the peasant will continually find excuses that at least work in his own mind. My parents' neighbour would habitually mow a strip of my parents' lawn, because they thought how other people mow their lawns was their business. (They cut it so short their own lawn was a dead brown more often than not.) Or rather, because they were spiteful folk. There's no point in confronting them about it. Even if their excuses could be short-circuited, they would simply find something else to be spiteful about. Have to intimidate them by credibly threatening (apparently-disproportionate) vengeance, which is of course illegal in the Regressive regime.
Modern bans on "murder" are bans on both insanity (which doesn't work, for some reason) and of vengeance.
P.S. I guess this means being a mob should itself be a crime? Peasants understand punishment. It's much safer to let folk gather if you're legally allowed to grapeshot them at any time if they seem moblike. You can make peasants reliably snitch on their parents with tactics like these, and likewise you can make them snitch on a mob rather than join it...which means you hardly ever have to bust out the grapeshot.