If such a police force were to see the light of day my recommendations would be:
1. Do not give them the complete powers of the regular police. Give them adequate power to detain the suspect, and any material which may be incriminating evidence, but such detention should not entail interrogation and should be no longer than the time it takes for them to deliver the said suspect, along with the evidence, to the local police station.
2. Volunteers should be vetted. They should be not less than 24 and not more than 55 years of age. Teenagers, children, women in labor, the elderly, people with past criminal records and the infirm should not be allowed to volunteer. Furthermore, the volunteers' credentials and identification data should be logged, their performance assessed by independent peer-reviewers and complaints against their character, behaviour or commitment to duty taken seriously.
3. The volunteer police force should have direction. It should have a code of conduct, clearly demarcated rules of behaviour and volunteers should be held to an oath. Their duty is to serve and protect the people of the neighborhood from crime and criminals in general and therefore they are not to fall short of observing the strictest of moral and ethical uprightness. Any shortcoming be it as trivial as cussing and being rowdy to serious violations of law and order through vandalizing property or carrying out punishments is to be responded to with sanction and condemnation. I can not stress the importance of this point well enough. Lynch mobs are also vigilantes and lynch mobs are not what we want running our neigborhoods.
All in all it's a good idea, and if done right it has the potential to transform ordinary 'bayhiss' citizens into responsible, contibuting, law-abiding individuals.